Was the 10.15 Serb Mauser ever used much in mainland Europe as a sporter round? I have a gun coming in that is a smoothbore .411 diameter. It is a single shot break open built in Belgium stamped 10.4 on the BBL which is .411. The serb Mauser uses a .411 bullet and the base of the chamber and the rim cut both measure just slightly larger that a serb mauser cartridge should. When the gun comes back in I will post pictures and markings for more info, but this was the only cartridge that even close to fits the dimensions of the gun. Just curious why a sporting rifle might be made in a smoothbore in this cartridge. The dimensions don't seem to fit anything as far as shotgun shells. I'm sure a chamber cast will tell me more when i get the chance to pour one.

Could also have been used as a shotgun with shot loaded cartridges.
I've got 41 Swiss (rimfire) cartridges that were factory loaded with birdshot (by UMC believe it or not).
Also 58 Musket, 47-70 and a host of other "shot loaded" rifle cartridges were made.

I was wondering if it were a shot loaded rifle cartridge. Just having a hard time finding ones that might match dimensions. When i get my hands on the gun again, i will know more.

This is from a discussion on another forum, and while it relates to the 303 Brit, you can see how some rifles were modified, and the same may be true of your 10.15 SM

The one on the left is a shot load, for the (modified) rifle below.

The Enfield smoothbore shotgun in .410 was also used in the Indian Prison Service , I believe .

I have not been able to locate any information re your particular rifle caliber , sorry.