Do you still need a buttcap?
Got an old G-3 carrier sitting around? Smear some Diamond dust lapping compound on the inside of the receiver and just run the carrier back & forth until it falls through by it's own weight. We're talking "slides easily". It won't take long... this stuff works quickly. Then when it's sliding good, scrub the interior of the receiver to get the remaining compound out.
45acp... turning human garbage into useful fertilizer since 1911
As you slide down the banister of life, may the splinters never point the wrong way.
A dog has the right attitude concerning life. If you can't eat it or screw it, just pee on it and walk away.