Does anybody know if the Mauser shotguns are 2-3/4 inch or 2-1/2 inch chambers and where can I find 2-1/2 inch 12 gauge shells?

By this time the chamber could be anything. Get a machinists scale and measure the chamber, then compare with a known shotgun chamber.
A 2 1/2 inch chamber is longer than 2 1/2
to allow the crimp to open.
Good Luck!

Is this one of the old bolt guns?

Cut the crimp half of a 20 gauge shell off and discard it. Take the remainder and insert it in an open 12 gauge shell with the 20 gauge brass sticking out (see photo). Insert it in the chamber and try to close the gun or the bolt. If it will close then it is 2 3/4" at least. If not it is somewhat less.

Here is a picture of a simple tool to measure shotgun chambers and chokes. I made this tool 40 yrs ago and have used it on 410-10 ga. with very good results. It is marked on the other side from the center pivot to the dial side so you can read out the measurement as the tool is slowly pull out of the chamber. This has made me many dollars to check chokes. It takes about 5 seconds to get the measurement. Les


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