I have one of the Mauser shotgun you refer to, my father purchased it for me some 50 years ago it's still in my cabinet. Quite well made actually, though the stock was rather plain and I restocked it. Mine is in 16 guage, i've seen several 12s but never a 20, the12s considered rather risky although i've never seen or heard on one coming apart. This is due to the fact that in the 12 the action locking lug recesses are machined out to allow the 12 to be chambered, in the 16 I believe some recess still remains. The 12 uses the third back lug in the lockup and the safety lug, normally hidden under the extractor when the action is open. I stay away from any "magnum" loads and have never had a problem. --- John 303