I have most of the parts to make precision AR-15.  I have a Colt 6724 Upper with a match trigger installed in a Colt lower.  I still need to buy a bolt/carrier etc. I have the following questions concerning the bolt.
- What can a gunsmith do besides headspace to make the gun more accurate?
- Can I just take the upper/bolt into a gunsmith for accurization work or do they need the lower as well.

Thanks

There is nothing your 'gunsmith' can do about adjusting the headspace once the barrel extension has been fitted to the barrel and the chamber cut. He might search around for a bolt that has slightly thicker lugs, which would tighten headspace minimally (half a thou or so, insignificant). The only way to get a minimum headspace is to install a new unchambered barrel, and then ream the chamber to minimum depth. The lugs on both the bolt and barrel extension are made to spec, and should all be the same and interchangable. Anyone that tells you they are 'adjusting headspace' on your existing barrel is screwing you (unless they have a good supply of bolts which they are trying with your barrel extension.) Accurizing of your existing unit is going to be limited to installing floating handguards and developing a handload that it likes. If you are limited to factory ammo, most of this discussion is redundant anyway. A good adjustable trigger makes it much easier to accuracy tune. Good luck.