I also have a German double in 9.3x72r that won't fully chamber factory ammo. Hangs up in a similar way. Turns out the chambers are super tight. I wrapped the brass in a single layer of household tape and ran that through the sizing die and it fits and fires.

Be sure to cast the bores as well as the chambers, as if I recall correctly, mine are also undersize/tight.

Rifle is quite nice. Retailed by Weisgerber, like a pre-war Merkel I have as well (that one in 8x57R with a .318 bore.)