Hello Hutch,
This is an Austrian made gun from Ferlach, nitro proofed in 1932. Shot barrel is 16g for the short 2.5" shell, rifle cartridge is probably 6.5x58R (S&S) or maybe 6.5x70R; a simple chamber cast (using a candle) will tell you the exact cartridge. In any case this ammo cannot be store bought but can be ordered from a specialist, or handloaded.

With kind regards,
Jani