Steiner carries a Russian PPSh-41 sub-machine gun originally chambered for 7.62×25 Tokarev, a round the Germans did not (officially) use, but the 41 was easily rebarrelled for 9 millimeter parabellum round, which the Germans did use, and the weapon was, in fact, adopted by the Germans as the MP717(r).
Whilst the drum magazine worked with 9mm ammo, the Russian curved stick magazine did not. If the Germans chose to use stick magazines with this gun, they had to modify the magazine housing to accept German issue MP38/40 magazines. However, doing this would prevent the use of the drum magazines. So Steiner’s weapon, therefore, may be in the original caliber, or may have been rebarrelled for 9mm parabellum, but had not been modified for German stick magazines.