If this is a factory original receiver and barrel, stop.

Disassemble bolts entirely.
No extractor or ejector-period. You cannot slip a hs gage into an assembled bolt and get an accurate reading.

Clean chamber with proper brush and solvent.
Then try to check headspace with both bolts.

If it is a new barrel and old receiver--have it checked by someone else before reaming.
few factory barrels were short chambered in my experience in checking dozens.

A little goes a long long way with a reamer, fast.