polygonal? land and groove? WHY?

[patriot93a3]

I need some wisdom from someone. Why does HK manufacture standard land and groove type rifling in the USP 9 and 40? My USP 45 has polygonal rifling, what's the deal?

[Big Bore]

Only the very early USP 9 and 40 had the standard rifling. Since late 94 all have the polygonal bores now. The .45 did not come out intil late 94 or early 95 IIRC.

[kassenz]

It is said the lands/grooves have a higher chance of tearing jacket lining. The Polygonal is cleaner, easier to clean, and offer higher velocities with less damage to the bullet jacket. Many people believe it is a more accurate rifling. It is used in the $10,000+ PSG1 sniper rifle.

It is believed to be a higher Quality technique, but harder to manufacture, and more expensive.

[jeray]

I have heard via several different sources including a gunsmith that conventional (lands and grooves) rifling is slightly more accurate than polygonal. HK and Glock use poly rifling to prolong service life and so than there is less carbon build up over time between cleanings, and to lessening the chance of a jam when being fired non-stop for a highly number of rounds (torture-tests). Both manufacturers specify that you are to use only jacketed rounds due to this type of rifling. Conventional rifling does not have this constraint. Both are good. Neither seems to be so benefitial that the industry chooses one whole-heartedly over the other. I think the PSG-1's accuracy comes mostly from the 25.6 inch length of the barrel, re-inforced receiver and total quality of build as opposed to the rifling.

[Scooter]

Polygonal barrels are more effecient as it provides a better seal allowing you to reduce the charge for the same velocity. That is one of the reasons why the Tubb2000 has switched to polygonal barrels.

I also think there's another reason why polygonal barrels are prefered. It's manufacturing costs. Polygonal barrels are all hammer forged. While conventional rifling can be hammer forged, broached, or buttoned. The dies, broach, or button can be easily examine to determine if it is still within specs. For example you lose the sharp corners on the rifling. But with a polygonal rifling there are no sharp corners, also harder to determine if it is within specs. So I bet you can a lot more barrels off of one die with the polygonal rifling.

[Unbreakable]

As said above polygonal bores tend to deliver more of your punch as they don't allow gas to escape through the grooves as the bullet rides the lands. I have noticed however that my polygonal bore guns (HK, Glock, Desert Eagle) tend to lose a little accuracy from 50 rounds to 100 and 100 out to 300... never shoot more than 300 rounds through my guns in a range session. Not a considerable change but the groups seem to widen as the bore gets dirtier and dirtier. At the very first I thought it might be the gun heating up and changing it's handling but even on a long day of slow shooting with little to no heat build up the polygonal bores seem to become marginally less accuarate the dirtier (ie the mores rounds fired) they get. I also considered that my hand might be getting fatigued and causing a loss in shooting ability but my standard bore pistols (Kimber, S&W, Colt, & Springfield) don't seem to have this problem. And many of them beat the hand up worse than a USP... So no fatigue concern. But then again most of my USPs after 300 rounds are more accurate than the other aobve mentioned guns freshly cleaned LOL. Never the less they still drop every round in a man sized target's chest at 30 yards no matter how many rounds you fire. I just hate seeing my groups get gradually wider and wider.

[Big Bore]

I don't want this to be taken as a flame and is not directed toward anyone, only I felt compelled to reply because IMHO there is just too much incorrect information being put out about polygonal VS conventional bores. I do not understand how people think that conventional land and groove rifling lets gas escape around the bullet. Throat erosion and gas cutting that occurs right in front of the chamber happens because of a combination of several things, not just the very tiny amount of gas that might be going around the bullet before it completely obturates, but from the burning of the powder and the tremendous heat and peak pressures that occurs in those first few inches the barrel. If gas were shooting past bullets all the way down the barrel, barrels would burn out in no time flat, accuracy would non-existent, and extreme spreads would be sky high. None of these events are seen in conventionally rifled bores any more so than they are seen in polygonal bores.
If you slug a conventional rifled bore you will find the groove diameter is the same as the bullet's diameter. And don't think for a minute that a jacketed bullet will not obturate to fill the bore if slightly undersized, say shooting a .308 bullet in a .309 bore. They will completely obturate and seal the bore the same as a soft lead slug will. We are talking about pressures here that will blow a barrel apart if obstructed; you don't think that it won't obturate something as soft as a bullet, hogwash. In a .45 ACP you shoot a jacketed bullet of .451 to .452 diameter. The groove diameter of the .45 ACP conventional rifled barrel is .451 but can vary as much as .001 inch in a crummy barrel. The land diameter is several thousands less than that. The bullet does NOT ride on top of the lands, the rifling cuts into the bullet. No gas escapes around the bullet any more in a conventionally rifled barrel than does a polygonal bore. Also, if polygonal bores are so much more accurate which I often hear stated, how come you do not see them showing up in all the BR guns and LR guns? You don't. People who shoot LRBR will pay any amount for a barrel if it will give them an edge, and polygonal bores do not. They are accurate, sure, but they are not more accurate than conventionally rifled bores. Sure, you can find an individual polygonal bored barrel that is more accurate than an individual conventionally rifled barrel, but that is purely anecdotal, meaning nothing to the overall picture. If polygonal boring were more accurate you would see Lothar-Walther, K&P, Lilja, McMillan/Wiseman, Shilen, Wilson...all the top barrel makers going to polygonal bores. They aren't. You don't get better barrels than what these makers produce, so while there MIGHT be advantages to polygonal bores on a combat rifle/pistol there are none when it comes to accuracy. That is just pure sales propaganda, period. Sure, one can find all the anecdotal "evidence" one wants to make a point such as my HK out shoots my Colt, but that proves nothing. Look at what the top barrel makers in the WORLD produce, and an "ain't" polygonal bores. And furthermore, if polygonal bores are just as accurate as conventionally rifled bores but offer less throat erosion, then do you think for one minute that if these top barrel makers could sell a barrel that is just as accurate but last longer without having to be set back and rechambered they wouldn’t? Of course they would because if it were true, a barrel maker that came out with a barrel that was just as accurate AND held its accuracy longer would put the others out of business in a heartbeat. In the world of LRBR shooting, what works is used, they will do or try anything to give them any edge at all, even if it means killing a chicken at midnight under a new moon. They hold few loyalties.

The other claim often made is the polygonal bores offer less resistance so they shoot the bullet faster. Again, hogwash. Take a bullet in a conventional rifled barrel. Shoot a particular load over a chrony and get a velocity. Now, take those same bullets and coat them with molly and shoot them over the same load in the same gun, what happens? Velocity goes down because of LESS resistance to the bore, and because less resistance means less pressure develops in the barrel. To get the pressure back up to where it was before, you add more powder. More powder means more gas expanding, and by the time you add enough powder to get the pressures to where they were before you have a larger volume of expanding gas and the end result is that the bullet is going faster with a molly coated bullet when loaded to the same pressure. That is why you cannot take a load worked up with molly coated bullets and substitute a non-coated bullet. The non-coated bullet will raise pressures sky high. So, if one sees higher velocities with a given load out of a polygonal bore versus a conventional bore, then it must be because HIGHER pressures are being developed in the polygonal bore, not less, so the polygonal bore must be creating MORE resistance to the bullet, NOT LESS.
I have shot loads though many .45s in polygonal and non polygonal bores, and the velocity difference between barrels of the same length are well within the standard deviation of individual barrels. One HK might shoot a hair faster than the conventional bored barrel, another HK shoots a little slower, same as with the normal variation found between any set of barrels.
I know HK and Glock make all kinds of claims about the superiority of polygonal versus conventional bores, but nothing I have seen proves any, and some of the claims made by supporters are against the laws of physics. You cannot decrease resistance which lowers pressures and increase the velocity of a bullet, it ain't gonna happen.
The only claim often made that could hold any water is that they last longer. IF the polygonal bore offers less resistance, then less pressures develop in the barrel, then of course that barrel is going to last longer, same as with shooting molly coated bullets in match barrels with conventional rifling.

IMO if there is any advantage of one method over the other it has to do with manufacturing costs. If anyone can make a barrel that is just as good as another barrel but make it cheaper, then that is what they are going to do. HK and Glock went the polygonal route and the barrels work. Why change. Same with all the others. They chose the other route, their barrels work, why change. I think what we see here are two methods to achieve the same end.
We’ve all heard raves about new barrels that come out from time to time, gain twist, tapered bores, RH vs. LH twist, more or less number of lands and grooves…and as yet has any one method proven to be light years ahead of the other? No, they have not. Marlin invested tons of money in their Micro-Groove barrels and they work. But yet they don’t work as well with all bullets so they have gone back to conventional rifled barrels for those who regularly shoot lead. And while with jacketed bullets the Micro-Groove works very well, it does not work any better, not when you get right down to the brass tacks of it. And don’t forget the gain twist and tapered bored barrels which are rare birds indeed. They work sure, but do not offer the hoped for results often claimed, but once invested, the makers continue to make them that way for as long as they can because as with any venture, there is a client base that will swear by this or that without any real evidence to support their prejudice (sort of like me with the .45 ACP). If any of them offered any real gain, then those barrels would dominate the shooting industry and the firing line, they do not.
In fact, the only barrels that do dominate the world of firearms IS the conventionally rifled barrel. Many other barrels are good, some very good, but other than the occasional anecdotal story, there is no solid, irrefutable evidence that I have seen that suggests any one method is any better than the other.
OK, now take all the above with a grain of salt. It is worth exactly what you are paying for it.

[Unbreakable]

Just to sign in on my above statement. I am no expert. I have read time and time again here and other places that the same loades from a Poly barrel chrono in at higher FPS than conventional bore. Is this true... I have no idea. I have accepted it as a possibility but like I said I am no expert when it comes to the science end of the firearms world. Big Bore seems to have more of an in depth view of this and as always I'd trust his view. I have no idea how fast or hard my bullets are coming out of each gun other than what the box claims. As for accuarcy... it's very true to say that My HKs are more accurate than my Colts is no substantial proof. I may just get along with the USP better. The only USP I have ever had trouble shooting accurately was the 45 Compact. I went through two of them and didn't like either one. But that can reasonably come down to shooter preference and ability. Losts of guys here rave about the Compact 45. I will, however, stand by my observation that my Poly bores seem to lose a hint of accuracy the dirtier they get. I have noticed no such deviations in my Conventional bored guns. To say I stand by it doesn't mean this holds true for every single poly bore out there. It may just be mine. It may once again be shooter error. But it is something that I have come to notice and can almost foresee when going out to a day of shooting. Today I took my 40 cal USP out to dial in my new adjustable sights. Took about 40 rounds to do to my liking. Then we spent the day shooting at everything from Stand Up steel targets to soda cans to cigarette packs. After about 280 rounds I was missing the cigarette packs more often than I was hitting them at about 50 paces and I just new it was because my gun was gooped up pretty good by then. And this is with a 3"X4" target... not the broad side of a barn. So like I said earlier we're still talking usable accuarcy. Poly vs. Conventional... I like Poly... My view is most definately lop sided as I will prefer what my HKs have as I shoot the best with them. I believe they do last longer... and other than that I choose my guns by name brands that I have experience with a trust. I shoot just as good with my Kimber BP Ten II as I do with my buddies $2,500.00 Wilson. That to me either means I am a good shot with my Kimber or a bad shot with a Wilson LOL either way I don't see where the other $2,000.00 comes in. And once again that means nothing. All that means is that my mind is pretty much made up that I shoot as good as I'm gonna within the $500.00 to $1,000.00 gun range and have no need for the super pistols. I trust Poly... I like poly, and that's what I suggest you go with. No science behind it or evidence to support it as I do not own the expensive testing equipment to proove one over the other. But that's my preference and I guess I'll have to live with it. Make your own choice.

[patriot93a3]

Thanks everyone for your input. I know I can always rely on intelligent information and insight from the members here at HKPRO. There is always someone willing to take a few minutes and add a reply. Thanks again

[tjloeb1]

Here is a link to a great source that describes rifling in great detail and shows the different types and how they are manufactured:

http://www.firearmsid.com/A_bulletIDrifling.htm

Tom

[kassenz]

Great link, thanks! More fact than folklore!

[Pharma45]

good stuff guys

[Scooter]

Polygonal barrels don't offer less resistance, but actually more resistance. Think of it this way, you are shoving a round peg into a square hole where the diagonal is the same width as the diameter. More pressure equals higher velocity, therefore you can reduce the load to get the same pressure and velocity.

The only top rifle shooter to use polygonal barrels is David Tubb. He now specs Schneider polygonal barrels on the Tubb2000 built by McMillan.

[Handy]

[ QUOTE ]
If gas were shooting past bullets all the way down the barrel, barrels would burn out in no time flat, accuracy would non-existent, and extreme spreads would be sky high. None of these events are seen in conventionally rifled bores any more so than they are seen in polygonal bores.

[/ QUOTE ]

Big Bore,
This line of reasoning makes no sense. Hot gas is what drives the bullet; it is present down the entire bore. Gas blow-by is not going to be any more errosive than the rest of the gas in the barrel. It doesn't take on any new properties because it is going through a narrow space. Throat erosion occurs because the throat is by the hottest gasses. Again, the gas doesn't get hotter because it is leaking past the bullet (if anything, it may be cooler.)

An examination of a conventionally rifled barrel after shooting reveals that the inside corner between land and groove has the most fouling. Yet you're implying that the bullet is able to completely conform to these 90 degree corners. Obviously, they aren't, or the bore would foul evenly (like polys do).


Also, the claim that top barrel makers would use polygonal if it was superior is also questionable. Barrel makers start small, and perfect their craft while building a reputation. You can't start small with polygonal barreling - the rotary forge is a bit pricey. And people like Shilen aren't going to make their reputation with cut rifling, then throw it away on some other technique when they get the cash flow. On top of that, buttons or cutters are cheaper than mandrels, which is important when your business turns out barrels in 30 calibers, instead of HK's 5 or 6.

Polygonal barrels seem to be about as good as the best of any barrel type. They seal better, but also cause more friction, so it is sometimes a wash, velocity wise. They are definitely one of the toughest barrels made, and the barrels of HK and Peter Stahl (a custom barrel and gun maker) are prized, when available aftermarket. Certainly a good land and groove barrel can be just as good, but I can't think of a polygonal barrel by any maker (seven come to mind right off) that doesn't have a reputation for intrinsic accuracy. So I would say that choosing a polygonal barrel is a very safe bet if long life and accuracy are expected.


In terms of cost, polygonal barrels, in mass quanities, are probably cheaper. The smooth mandrels do not wear out as quickly as buttons or cutters. But Glock, the ultimate in cheap fabrication, would not have chosen a more expensive production method as a public service.


Everyone should keep in mind two things:
1. There is more than one way to skin a cat.
2. The gun industry (and gun buyers) resists change with all its might. The fact that polygonal rifling is only associated with upstart companies should tell you something about how ideas are excepted in this industry.

[Big Bore]

On some thing you and I agree completely. I said throat erosion is caused by pressure, heat, and expanding gasses, not blow-by.
You said, "It doesn't take on any new properties because it is going through a narrow space." but it does.
Hot gases blowing by steel under pressure will cut steel in a heartbeat (like a cutting torch) and to see evidence of this all one has to do is look at the gas cutting that happens to the frame above the cylinder gap in revolvers. Gas blow by would cut the steel barrel to pieces in no time flat if that happened, which it does not.
The bullet does not have to conform to the 90 degree edges. The rifling CUTS those edges. The bullet obturates to the bore diameter in the throat and then the rifling lead and rifling cuts the grooves into the bullet. The bullet is not molded into the groves as it is with the polygonal bore, those grooves are cut into the bullet as if you were using a engraving tool. Look at a fired bullet shot from a normally rifled bore, at the rear end of the bullet you will see the metal jacket extend very slightly, as if pulled away, behind the base of the bullet, just as if you had used a cutter to cut the groove from nose to base, because that is exactly what happens. It is true that the trailing edge of the rifling fouls more than the leading edge just as you said but that is because the leading edge gets the most wear and does the most cutting. The trailing edge of the rifling does not get anywhere near the friction or pressure as the leading edge and fouling does tend to build up there first as it is not being stripped away as much as it is on the leading edge.. That is also why the leading edge of the rifling wears the quickest and will be rounded over long before the trailing edge is. However, you said poly bores foul evenly, but they do not. If you look at a polygonal bore you see that copper washing is not even over the entire surface of the bore either, just as it is not with conventional rifling. Otherwise you would not see copper streaks in the USPs bore (which you do) but rather the entire bore would be painted, which it is not. I’ve seen way too may copper streaks in the bores of my HKs to even entertain the idea that they are coated evenly. That just does not happen.
You said: "Polygonal barrels seem to be about as good as the best of any barrel type. They seal better, but also cause more friction, so it is sometimes a wash, velocity wise. They are definitely one of the toughest barrels made, and the barrels of HK and Peter Stahl (a custom barrel and gun maker) are prized, when available aftermarket. Certainly a good land and groove barrel can be just as good, but I can't think of a polygonal barrel by any maker (seven come to mind right off) that doesn't have a reputation for intrinsic accuracy. So I would say that choosing a polygonal barrel is a very safe bet if long life and accuracy are expected."
We agree on that almost completely, but I still say if the best polygonal bore was MORE accurate than the best conventional bore then you would see a lot more polygonal bores in the high dollar LR accuracy barrels, and winning LR target barrels are conventionally rifled by an overwhelming majority.
And, I never said polygonal bore were no good, only that they were no better. When it comes to competition barrels, if polygonal bores offered any real accuracy advantage (and that includes holding their accuracy longer) then all the top barrel makers would be using them. Shooters would demand them and all it would take is one person to start cleaning house regularly with a polygonal bored barrel and the change would be almost immediate. As I said before, LR accuracy shooters have no brand loyalty, if someone builds a better mouse trap they will beat a path to their door. Eight years ago Leupold Mk 4 scopes dominated LR .50 BMG shoots and it took less than two years for almost every one of the top shooters to switch to Nightforce. Same holds true for barrels or anything else when it comes to LR accuracy shooting. While mass producers like the firearms companies might resist change, the top barrel makers who make their living catering to the accuracy obsessed would go broke if they did not turn out the most accurate barrels they can, and if it meant switching from conventional rifling to polygonal, they would in the blink of an eye. Their livelihood depends on it.
Also, as I said in my first post " I think what we see here are two methods to achieve the same end." so we agree that there is more than one way to skin a cat.
But I don't think that Glock and HK are upstart companies, only that they chose to use a different method because of what they consider advantages
Again, I said in my first post: "if there is any advantage of one method over the other it has to do with manufacturing costs. so we agree on this also.

[wahodges]

You guys should write books. Seriously, there is such a wealth of knowledge here. My friends never read all this stuff and are always amazed when I pull some factoid out of my behind and it is knowledge from the members above that make it happen! Keep it up!

[patriot93a3]

Thanks again guys. I learn something new everyday. The wisdom knowledge and experience are unmatched here at HKPRO.period

[Handy]

Big Bore,

Gas blow by allows gas to expand. Expanding gas is cooling gas. Gas escaping around the lands would be cooler than the gas in the bore. How is it that expanding/cooling gas is going to cause more erosion than the rest of the gas pushing the bullet? Gas FORCED through small holes does heat up, but gas leaking through the same cools down. I'm not sure who would have told you that the escaping gas would be erosive, but they aren't looking at the whole equation.

I already stated why bench rest shooters and the like aren't using many polygonal bores: How are they going to get the customized products they buy from a company only set up to produce a few calibers/twists? It is not at all economical to have 400 different mandrels made just in case Joe wants his 6mm in 1 in 13 twist.

Plus, how is a benchrest or whatever shooter ever going to decide to use a polygonal bore if there are no guns already to try out? An HK 770 might be quite accurate for a semiauto, but that doesn't translate into data to suggest trying its bore type in a completely different kind of target gun.

I think the fact that even ONE noted competitor is going to the trouble, despite the aforementioned obstacles, speaks loads about their utility for accuracy. Better accuracy? Who knows. No one has really built match rifles out of polygonal barrels before. The world was flat, until it turned out not to be.

[Big Bore]

Then how do you explain gas cutting that happens on a revolver frame. Same principle. The gas that would blow by the bullet would be burning gas forced through a much smaller opening under intense pressure which would cut the barrel the same as it cuts the frame of a revolver as it escapes through the cylinder gap, and that is a very well known phenomenon among all revolver shooters. We will have to agree to disagree on that one.

And I explained BR shooters too. If polygonal bores offered any intrinsic accuracy or longevity advantage, BR shooters would insist on them. Someone would supply them. How much more would it cost to make a hammer forged polygonal bore rather than a hammer forged conventional bores, all it would take is a new mandrel. All it would take is one company to produce a polygonal bore and they would put all the rest out of business if the polygonal bore offered any advantage. I stand by my explanation. Supply and demand. If customers demanded it, makers would supply it. They don't, so they don't.
You said yourself that there were makers of polygonal bores. I am sure they have been tried on BR guns. EVERYTHING has been tried on BR guns. Those guys are relentless for that last .001 inch and P bores have been around a long time. Trust me, they have been tried by multiple people. If gain twist, tapered bores...have been tried, P. bores have been tried. Why they don't dominate is likely either they are more expensive (but they may be cheaper to make, I don't know), no advantage, less calibers as you said...or a combination of them. But I will bet dollars to doughnuts that a lot more than one competitor has tried them.
And there have been bolt action rifles made with polygonal bores, I have seen them complete with their hammer forget outside twist pattern barrels but I cannot remember the maker's name right now, and it was reported to be very accurate. Again, I have never said they were not accurate, just that nobody has shown me, or apparently anyone else that they are any MORE accurate intrinsically than a conventionally rifled barrel.

I'm afraid that neither of us is going to change the other's mind so time to let this drop. No since in us saying the same thing over and over.
I will say, it is nice to be able to disagree about something and not have a flame war erupt.

[Handy]

Gas cutting in a revolver takes place just in front of the chamber - the same place that throat erosion occurs - where the hottest gases are located.

[Scooter]

There's one person we should ask that will resolve this issue, David Tubb. He's only won 11 highpower rifle championships. Now he has switched to using polygonal barrels. The big question is why? He's not afraid to experiment, how many people do you know use a $1200 powder measure.

[Big Bore]

And the same thing would happen IF gases were going around the bullet. Gas going past a bullet would be under extreme pressure the same, and even more so, than at the cylinder gap. Now, once the gas got in front of the bullet it would expand and cool because the bullet would no longer be obstructing the gas and it would expand and cool, but not until it gets in front of the bullet. If the gas is hot and under pressure as it would be if going around the bullet, it will cut, just like a cutting torch and just like the cylinder gap, there is no difference. The reason it does not cut in suppressors, brakes, or compensators is because the pressure is being released as the bullet is no longer blocking the bore or applying back pressure. In breaks and compensators the bullet has in affect left the barrel and the gas is expanding hundreds of times, hitting the baffles and being redirected all the time the pressure is dropping rapidly and the gas is cooling. The same happens inside a suppressor, the gas is being allowed to expand and cool and the whole time the pressure is dropping, that is what suppresses the sound of the muzzle blast, but all this is happening while pressure is decreasing, not increasing or being maintained. Bullets, polygonal or rifled bores, seal the bore and gas does not shoot past them once obturatted, if it did, the gas would cut the bore in very short order and the barrel would be eroded.

Some very interesting reading, not exactly pertaining to our discussion but very informative: http://www.snipersparadise.com/tsmag/june2001.htm

[bordercop]

[ QUOTE ]
... I explained BR shooters too. If polygonal bores offered any intrinsic accuracy or longevity advantage, BR shooters would insist on them. ...

[/ QUOTE ]

I'm not sure if there's any advantage in Polygon over conventional, for longevity. However, the sharp edges of conventional DO seem to give more surface area for erosion to take place. THIS is the reason why Dan Lilja (anyone who even thinks about shooting BR, knows who he is) is working with cutting only 3 grooves instead of the usual 6, and so far it appears that he has extended barrel life by around 40%
He's still in the devlopment and testing phase, but so far everything seems to be going as he thought.

[Handy]

Sorry to dredge up this old thread, but in regards to gas blow-by: If this would cause extreme erosion, are VP70 barrels wearing out at an incredible rate? They are designed to let gas escape around the bullet.

I'm embarrased I didn't remember this at the time, but it is still relevent.

[muffin]

excuse me! gentlemenly, logical thought provoking, informative discussions have no place on internet boards, you should be ashamed of yourselves....... oh wait i forgot im on HKPRO, sorry
thank you for this informative discussion, ive seen some ignorant disgusting uncivilized flaming, arguments on boards and im glad hkpro is not all one of them

i wanted a serious evaluation of poly vs. button-land groove
comparisons since i thought poly is supposed to be better??? maybe???? my usc is poly, right? and ive noticed copper residue on the lines after solventbrush cleaning, the same on my ptr land and groove. however im happy to report the sl8 can be cleaned to new chrome perfection so the lands and grooves are indistinguishable. any thought??
id like to have impossibly clean guns and again thank you for the informative discussion
have fun!! /ubbthreads/images/graemlins/cool.gif

[Big Bore]

The PTR is polygonal rifled also (and so is the USC). Even the polygonal bore will streak a bit so a little CR-10 (or other copper remover) may be in order. It doesn't take much to remove it though.

[rpm]

The one thing I like about the poly is the glass look after cleaning looking down thru the barrell when its dry.I know , I know easily amused, but you can easily SEE anything thats there.

[Handy]

BB,

No opinion on the VP70s?

[Big Bore]

The 9mm is a very low pressure round. I would not think you would see much if any errosion in it for a very, very long time, not like you would in a .454, .500, or .475 handguns or most any rifle cartridge. Plus, I am unfamiliar with the VP70 pistol operation and do not know for a fact that the pistol is designed to have gas blow by the bullet. That seems very odd to me.

[muffin]

bigbore the ptr's to the best of my knowledge now have land and groove. at least mine does from sarco dealer sale 650 and transferred to a total of 765
the word i read was that to many people had problems shooting military crap, extracting and what ever, the chrome poly are out and the unchrome land and groove is what i got so we could shoot affordable crap instead of premium american.go to sarco to read the exact reasons listed instead of my memory
thanks for the input

[Big Bore]

Really? Mine has a serial number in the low 100s and it has the polygonal bore. It also had the extraction problem that they fixed with a rechamber. I do notice on their web site they have dropped reference to the polygonal bore on the PTR-91 and PTR-91 A1 and in fact on the T, K, KT, and KP models they do mention the button rifling.
You got me wondering now so I checked my owner's manual. The owner's manual that came with the rifle clearly states that it has polygonal rifling. However, the owner's manual that came back with the rifle after the rechamber clearly states button rifling. I know it had polygonal before the rechamber but what about after? Did they change my barrel or just rechamber? So, I checked that also and mine still has the polygonal bore. Since the rechamber it has run 100% with Australian, Black Hills, WWB, and some other mil-surp I got so I guess it doesn't matter much with the correct chamber. Thanks for the heads up on the change because since I got mine running I have not been paying real close attention to the JLD line.

[Handy]

BB,

First, 9mm is the second highest pressure combat ammo, right after 10mm. +P ammo runs 37,500, standard 35,000. .40 is limited to 35,000; .45 +P is 23,000. Even .357 and .44 Magnum only go to 35,000 and 36,000 respectively.

The VP70, being a blowback gun without a very heavy slide, uses gas blow-by from extra deep grooves to lower blow back pressures (and velocity) to a more manageable level.


So if gas blow-by is highly damaging, and the VP70 is allowing peak pressures of 37,500 to squeeze through its grooves, are these barrels burning themselves out, as you said earlier?

[Big Bore]

35K is not high pressure compared to rounds such I mentioned which all operate at 50-55K psi. 20K psi is a huge difference in both pressure and temperature. I don't know at what pressure gas blow by can start causing serious cutting but it does happen, that is not even really disputable. Hi pressures and temperatures are what cause throat erosion and cylinder frame cutting in revolvers. While it happens to a much lesser extent in the .44 magnum and .357 Magnum, it does happen. All you have to do to prove that is look at the top strap of most any revolver and the higher the pressure the more severe the cutting. When you get to the .454 and like pressure rounds gas cutting can become a real problem unless special steps are taken to prevent it such as minimum flash gaps and special steel used in that area. Smith and Wesson puts in a special steel shim right above the cylinder flash gap in many of their revolvers.
In closed chamber rounds like rifles gas cutting occurs primarily at the throat and lead area of the chamber/bore where the pressures are highest and when the bullet has not obturatted to the bore. Yes, even jacketed bullets obturate to seal the bore even as the rifling is being cut into them. Once the bullet obturates gas blow by is minimal to non existent and with the reducing pressures as the bullet travels down the barrel gas cutting does not occur or occurs at a much lesser extent. If there was a large amount of gas blow by caused by the cutting of the rifling into the bullet OR the bullet not obturating to the bore there by sealing off gas blow by, gas cutting would occur much farther down the barrel than it does. And, if gas were blowing by the bullet willy nilly then the internal ballistics of the round would reveal large spreads in velocity from one round to another, much larger than normally seen and if much blow by was happening long range accuracy would be almost non-existent.

I would only have to venture a guess that the VP70 does not experience a debilitating degree of gas cutting because the pressures are relatively low and combined with if gas is blowing by the bullet that is certainly lowering chamber/bore pressure and reducing velocity as you mentioned. I would like to see chamber pressure readings of the VP70 with a 35K psi round. With the gas blowing by the bullet I suspect that pressures are considerably lower and the lower the pressure, the less heat, and with less heat and pressure less cutting occurs by gas blow by.

[Handy]

The chamber pressures are the same, since the blowby can't occur until the bullet leaves the chamber and enters the rifling.

[Banjo MD]

I will say, it is nice to be able to disagree about something and not have a flame war erupt.
One of the best discussions I've ever read and one of the most civilized, too!
My hat's off to all of you and thanks.

[Sobriquet]

Holy necropost! lol

[Banjo MD]

Holy necropost! lol
Wow! Never looked at the date! Still good reading, though.

[Jimster58]

Another point that I didn't see addressed in the wealth of information and spirited debate here...the presumed gas blowing by the seal created by the bullet in the bore would have a greater velocity than the controlled expansion and velocity of the gas remaining behind the "seal"...maybe a very significantly higher velocity...and theoretically, therefore, have more capacity to cause bore erosion. Higher temps would certainly be a factor in facilitating erosion, but where is the breakpoint where velocity is outweighed by the energy of a hotter gas? Anyone have any real-world experience that can be analogously applied to the "gas-blowby hypothesis" and it's erosive properties?

[mustang400]

Its always good to hear from Big Bore, he cleans up the mess and moves on!!!

[richark1]

"Um, can you repeat the part about the stuff when you talk about the thing?"

[Omega Man]

Which is more accurate?

[AviatorDave]

Which is more accurate?

First of all, what happened in 2006 and 2007? This thread was going strong in 2004, 2005, 2008, and now 2009. But 2006 and 2007? Crickets.

Anyways, in a pistol, neither is more accurate.

If you look at events where accuracy reigns supreme and money is no object, like benchrest shooting, everybody uses land and groove rifling.

Polygonal rifling's claim to fame is very slight increases in velocity due to better sealing, ease of cleaning, and longevity. And the advantages to a manufacturer may be increases in production volume.

[jpoulette]

I just stumbled across this post - thanks for the information. AviatorDave basically distilled what I've heard.

Any post going strong this long is a beauty!

[.45ACP]

I'll throw my hat into the ring.

I've chrono'd the same loads out of a USP 45 Fullsize (4.41") and a 5" 1911 and found the USP to average 20-30fps more. I've seen other people post the same results here, one guy even tested a USP Compact (3.8") against a 5" 1911 and the results were close.

[SIGtrarian]

Which is more accurate?When talking about handguns, I would say yes, they are more accurate.