I believe we will need some additional information on your barrel.

1) Did the barrel come with the barrel extension installed? If yes, generally these barrels are headspaced by the manufacturer unless one slipped through.

2) What is the chambering, length and twist on the barrel? .223 Rem, 5.56mm or 5.56mm NATO, 1/7. 1/9, 1/12, 1/14, gain twist, etc.

3) Is your chamber or bore chrome lined? It gets difficult to finish ream a chamber that is chromed plated.

4) Was the barrel advertised or marked as short chambered? I have heard of match barrels coming in short chambered, generally these are sold to gunsmiths who are building match rifles for a customer.

If the barrel is one that was short chambered you can rent a finishing reamer to get the proper headspacing. I would highly recommend renting a set of headspace guage as well to ensure the headspacing is proper. Kept in mind that 5.56mm NATO has a chamber pressure of about 60,000 psi.

Cheers

--fjruple