I have a Mauser rifle and can't figure out caliber. Not 8mm or 7.62 or 30-06. Has custom Julius Hessmer barrel?

My grandfather left me this rifle when he ped. It has a custom Julius Hessmer ( Hoflieferant Berlin ) semi octagon barrel with ridge. It also has custom "jewling" on the bolt action. I don't want to pay for a casting as I figure someone has to know what it is. The local gun shop is also researching it. The only other markings are MOD 98 and Z 234 although not on the actual barrel. Thanks!