You want to go carefully here. It may be a 6.5mm wildcat. You can not assume that it is 25-06.

A positive ID can be made using a chamber cast. You can also slug the bore to determine what bullet diameter it uses.

The fact that the barrel has no markings means that it was somebody's garage project, since federal law requires re-barreled rifles to carry caliber and cartridge markings on the barrel.