A friend emailed me a link to this thread and asked me to offer some insight. FWIW, I've been in the firearms industry for about ten years, including three years working as the Law Enforcement Operations Manager at Beretta USA and five years as the Federal/Military Projects Manager at SIGARMS (now SIG-Sauer). As such, I've dealt with FFL/shipping issues on a daily basis for a very long time.
It seems there are two confusing points here.
First, HK serializes a number of different parts. This is due to foreign regulations and has nothing to do with the discussion at hand. For the purposes of shipping a firearm within the US,
one part is declared the serialized firearm. For pistols, that is almost always the frame. (cf., SIG P250)
Second, again at least insofar as the immediate issue is concerned, the
shipper need not be an FFL holder but the
recipient must be (the recipient is then also allowed to ship it back to the owner, unless specific state law says otherwise).
So you can ship your barrel & slide anywhere you want. But if you ship the serialized frame of your HK pistol to someone who is not an FFL for the purpose of having it 'smithed, you've committed a crime.
Stay safe ...
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Todd Louis Green
pistol-training.com