Stop in for a cup of coffee

Well today is yet another day of sighting in folks deer rifles. Most guys have not touched their rifles since last deer season. Now with modern gun deer season on the horizon they have a crisis of needing to sight in their rifle! I charge 25 bucks a rifle. What’s not to like, I get to shoot all kinds of rifles, burn somebody else’s ammo and get paid to do it! I am doing them all right handed this year due to the shoulder surgery on left. Of course I am strongly left eye dominant but hey they will never know! They will hit whatever they put the cross hairs on and be thrilled!
Ok so here’s a riddle for you to solve.

Stopped at a gun shop yesterday in Kentucky on the way home. They had an 1898 Gewher Mauser. Original rifle but had been rechambered in something that they’ve not been able to figure out. Has a 7.62mm throat. However, 8mm Mauser, 308, 30-06, 7.62x54R, 7.62x39, 7mm Mauser , none of those fit or even chamber.