You could have a small chamber...the last one cut after the reamer was reground, worn or minimum SAAMI specs.
It does sound like an undersize chamber.
Will it chamber a sized case, no bullet, easy or hard?
If it were mine I would make a chamber cast first to determing the actual chamber size or use a small hole guage to measure it.
ANY machine shop worth it's salt will have pin guages that can be used also to get the OD at the mouth of the chamber or inside mikes to measure with.
If those number come up kraps I would send it back...
I agree with WT...I've seen very FEW undersized factory chambers.
Luck