If Der Ami is correct, you need to know ASAP. If 8mm-06, you should be able to chamber a .30-06 case. If it fits or comes very close to fitting, the rifle has been rechambered.

Also, look at the bolt face. If the .30-06 case fits properly against the bolt face, that is another clue.

If the .30-06 case does not fit, next try a .30-30 Winchester case in the chamber and against the bolt face. Sometimes 8.15 rifles were rechambered for .30-30 or .32 Winchester Special.

Many who play with 8.15 rifles make the cases from .30-30, .32 Special or .32-40 brass. The conversion process is simple.