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JJM, glad to see you on the Forum. For you guys who don't know, JJM is a wealth of information on U.S. Military shotguns.

So I'm clear on this, the punch marks are hardness test. And the P...

JJM, what about the punch marks found on some 520 trench gun barrels and receivers? I thought the punch marks are maybe hardness tests whereas the extra P proofs marks are because the gun had to be...

My observation on the Military 520 trench guns is that there are many in the 68,000 serial number range that have the extra P proof marks on top of the barrel and receiver. And in addition to the...