I just aquired a brand new Bushmaster upper less the bolt and carrier.  Can I just buy a new bolt and carrier and put in this upper and not worry about headspacing it?  

I have heard both yes you can just buy a new bolt and carrier and put them in your gun and then hit the range and also being told that you must headspace a bolt and carrier for the new upper.  

Any help would be appreciated on this one guys, I don't want to damage my new upper.

I [b][red]HIGHLY[/b][/red] recommend checking the headspace on it.

the cost to do yourself is minimal, considering the risk involved, you could say it was "priceless".


A competent gunsmith could do this for you if you don't want to do it yourself, or can't hook up with someone locally.

I got a new upper and want to use my old bolt/carrier (600rds).  I can't find anybody in CT to help me.

If you are concerned about headspace, try this as a quick check.  I often use a "Mexican Headspace Gage" to do this.

Buy a box of match ammo and select three.  I use a gage that clamps on a dial caliper that compares cartrige length on the mid-shoulder.  Don't recall the brand, but they are in the stores.

Set up the match cartridges as your zero.  Add thicknesses of tape to the bottom of a cartridge and measure how much length your cartridge gained.  Gently ease the bolt shut (use your fingertips, not the mainspring!) on these cartriges and look for increasing resistance that tells you you are compressing the cartridge.  Oh yeah, remove the extractor
and ejector before you do this.

This method will give you a good idea of how consistant headspace is with a known standard cartridge.  Use your judgement on proceeding if headspace is wildly out.

Of course, if headspace is too small you have another problem.      

Problem with the tape on case method is that both the tape and brass will compress so you're just measuring the compressibility of tape and brass.
For those who can't to have it checked or buy a real gauge, use the "Cheapo Headspace Check".  Just fire a factory round with the gun as far from critical body parts as possible and examine the ejected case for signs.  The AR family is pretty good on kabooms; just make sure the mag well is not above your head.  The fire test is better than the false security the tape method provides.