Peter Hambrusch (from the top rib) was born 1889 in Ferlach, and opened a shop in Steyr by 1919. This gun was probably made from Ferlach components.

Considering relatively light construction - no Greener crossbolt, back action locks, lightweight barrels -, the nominal 8.0 mm caliber was likely one of the low pressure group: 8x58R Sauer, 8x57R/.360, 8x72R. As Mike wrote, the bore should fit the .318 bullets.

Cheers,
Jani