This is often suggested but seldom demonstrated, so.....

I got around to casting the chamber on the Unknown Caliber Mauser in this thread: http://forums.accuratereloadin...9411043/m/4081097942

Here is the sequence that I used.

Setup: I stood the barreled action up with a rod inserted in the bore and a patch plugging the bore at the end of the chamber.
IMG_0313 by William Jacobs, on Flickr

My funnel is made from a soda can. I use this to keep Cerrosafe away from places that it doesn't need to be into.
IMG_0314 by William Jacobs, on Flickr

It doesn't take much to melt Cerrosafe.
IMG_0317 by William Jacobs, on Flickr

Casting the chamber
IMG_0318 by William Jacobs, on Flickr

After cooling for a few minutes, I knocked the chamber cast out by tapping on the rod in the bore IMG_0320 by William Jacobs, on Flickr

Just as we suspected, the gun is chambered for the 7.65 mm Mauser cartridge.
IMG_0322 by William Jacobs, on Flickr

This is the first "cigarette rifle" that I have run across in 7.65 mm.

Bill Jacobs

You have a rare bird! When I saw your first post about the 7.7 marking, I remembered a German combination gun that I had many years ago. It too, was marked 7.7 and was rimmed, so I was hoping it might be a 303 Brit. I did a chamber cast and it turned out to be a 8x57jr . I figured that the odds on your gun would favor the 8x57J, but the cast tells the tale. Lots more .311-.313 bullets available than .318. Thanks for the update!

Bill, the owner of that rifle is a very fortunate fellow.

That's a beautiful rifle in a dandy caliber. The 7.65 was ahead of its time in many ways.