I'm sure your rifle is not chambered for 8X57 Mauser, but a rimmed cartridge and my guess is it is a 8X57JRS (.323 bore) because of the stamping of 7.9, instead of 7.8 (.318 bore). The rimless Mauser ammo will not head space in a rifle chambered for the rimmed rounds. DO NOT SHOOT 8mm Mauser ammo in this rifle, till you find out for sure it is chambered for it, and I doubt it is.

This type of combination gun is properly called a "CAPE GUN"! A side-by-side rifle/shotgun with the rifle barrel usually on the right barrel. I would say that even not seeing this cape gun, the 7.9X57 normally indicates a 8X57JRS (.323 bore) but as 400 says you need to slug the barrel to make sure The 8X57 JRS, and the 8X57JR are both available from Sellier & Bellot , and both are rimmed cartridges. The 16 ga is the most common shot shell in Germany. If the shot barrel is chambered for the short shell, it is no problem opening it up the 2 3/4", and the choke in the shot barrel is historically full full choke, and needs be opened up to modified for modern shot-cup ammo. Most have the rifle sights cut for 196 gr rnd nose bullets.

I have a 16 Ga/8X57JR (.318 bore) cape gun, made by H. Berella of Berlin, Germany in about 1900. It is a side lock hammer gun with 26 inch barrels. This cape gun will place a 196 gr Norma soft point on the sights at 100 yds every time, and the shot barrel will place a Brenneke slug right beside that rifle bullet at 100 yds evry time, and with the shot barrel opened up for 2 3/4" , and modified choke, will take passing doves and fleeing quail with monotonous regularity.

The word Suhl is not a brand but the town where it was made in East Germany, the brand is Simpson! A sister to Merkel also made in Suhl. Your cape gun is worth the little trouble it takes to get set up to load for it, with a set of RCBS dies, and when opened up for American shot ammo will use good factory shot shells and Slugs. These guns are deer, elk, moose, and black bear guns, and the shot barrel will back the rifle barrel up with a slug, or shoot anything form clays to geese! Cape guns are a real hunter's gun!

.......................Enjoy