Thread: A gunsmith question - head spacing.

I have a wee dilemma, I think I might understand the problem but would like some expert advice. Have a standard factory REM SA along with a standard REM factory bbl in 260 cal (new or near new) bought bbl from some one on here a while back. So sent off to get bbl attached to action via local gunshop. Also provided a unfired brass Lapua case for headspacing. Msg back from gunshop says “smith needs a 260 reamer to do the head spacing”. Hence my post asking if anyone has a reamer I could buy or borrow.
So why would a reamer be needed when you’ve got a standard factory action & bolt, and standard factory bbl chambered in 260 ?
I guess that within tolerances for action, bolt and bbl chamber you might end up with slightly undersize headspace once bbl is screwed in fully ? Hence need to ream chamber deeper by a couple of thou ? Am I on the right track ? Or would there be other reasons for needing reamer ? And, if I can’t locate a 260 reamer, are there other options ? Would a 243 reamer work ?