This might be a dumb question but I was talking to a guy today who said that if I were to get a new upper for my Bushmaster that was from a different manufacturer, say An Armalite or one from Model 1 sales, that I could not use my Bushmaster bolt/bolt carrier in the new upper because each manufacturer makes them to fit their upper and there are slight differences that could cause problems. I would have to get a new bolt with the new upper. Any truth to this? Thanks.

Bolt carriers are interchangeable.

Bolts are not.

On an AR, the headspace is determined by the fit of the bolt in the barrel extension. There is no way to adjust headspace short of replacing the bolt.

And he is right, each manufacturer sets their own tolerances for each part, so their their bolts will usually work with their barrels ... but mixing manufacturers creates a potential for "stacking tolerances" that can add up to excessive headspace and blown-up receivers.

Now, you may be lucky (do ya feel lucky?) and a bolt from (B) may fit a barrel from (A) to give you correct headspace. But you should check it before you shoot, and if it's excessive, you have to keep buying replacement bolts until you find one that fits.

Or you could just buy a new bolt with each barrel, which is what I do.

As tony_k posted, bolts are not indiscriminately interchangeable.  A quality bolt, in new or good condition, should (that's [b]should[/b], not [b]will[/b]) headspace properly with any quality barrel in new or good condition.

It's cheap insurance to check the headspace any time you change out a barrel or bolt.

99% of the time you can slap any old bolt and barrel combo together and everything will be just fine, but it's your face, your hands, and maybe your life riding on that possible 1% mismatch.  The risk increases with especially worn/well-used bolts or barrels, or potentially suspect parts from unknown manufacturers.

Do spend the $15-$20 on a headspace gauge and check any new combination you try.  And it's not that much extra to simply dedicate a bolt to a particular barrel and never have to worry about it.  If you're that tight for cash, just get the headspace gauge and check the new combo, and if it's OK, you'll be OK.  Check periodically (every 1000 rounds or so), and anytime you swap back.

So if I did get an upper receiver from a different manufacturer from the lower, I should go ahead and get that manufacturer's bolt too and I should be fine? There will be no problems with putting it on a different manufacturer's lower?

I disagree.



True.



They do?  Most AR manufacturers dont even MAKE bolts or barrel extensions.  They buy them.  Typically from the same few manufacturers.

The parts are made to an industry spec, and they are assembled and headspaced to an industry spec.



An in-spec barrel *should* headspace fine with ANY manufacturers bolt, and vice versa.... since they dont make bolts.



It's not mixing manufacturers that *could* cause this.  Its mixing out of spec, or worn parts with other out-of-spec, or worn parts that *could* cause a headspace issue.



Good advice.



Mee too!  But I didn't always, and never had an issue.  If I was buying a used barrel, and a used bolt, I would definitely check it... and it never hurts to check ANY rifle you assemble or use.....  

But the bottom line is, they CAN be swapped, and different manufacturers makes no difference.  All that matters is the parts used are in spec, and not worn..... and therefore, that they headspace properly.

Tac45 --

The upper receiver isn't part of the headspace issue, nor is the bolt carrier.  The bolt and barrel extension determine the headspace and if they're made to spec there are no issues, other than excessive wear on the locking lugs, in swapping bolts.

This assumes in spec parts, not parts off the the tailgate of a truck at the local gun show.

If in doubt, and always a good idea, check with with the M16 [i]gage, headspace[/i].

Excessive headspace will not blow up your rifle unless you've used a grinder on the lugs and General Hatcher did that and couldn't get a rifle to self destruct.

-- Chuck

I would not hesitate (and don't) to buy a barrel from any of DPMS, Bushmaster, Olympic Arms, Armalite, and other top suppliers, and then a bolt and or bolt carrier assy from any OTHER of these same suppliers, assemble them and go shooting.

I have, and have never had a headspace problem.

Just for the record, last year I bought 4 well used GI bolt assemblies. Three were Colt marked and one was a contract (H&R.GM?) piece.  All were M16A1 take offs.  Why? 'Cause they were $15 apiece [:D]

Three of the four haeadspced just fine in all four of my ARs, and the fourth, while it closed on the "Go" guage would not close on the "Field" guage---and that one was so old, it still had a white insert in the extractor spring; and they went to the blue ones about 1967.  Put HD extractor springs in, mounted two of them in two of my ARs, and stashed the other two in my parts box.

Assuming a properly built barrel, use any durn bolt you'vr got. If the bolt is well used, as mine were, I'd check the headspace; but new, or nearly new, I wouldn't bother.

I have a Colt Barrel assembly with a Colt bolt (marked MPC) on my M16RR.  I brought it to the gunshow to buy a spare bolt (thought I'd headspace when I bought it just to make sure).  A person from J&T Distributing tried three bolts marked MPC and all three failed with excessive headspace.  We then gauged my original MPC marked bolt and it was fine.  At this point, we tried an FN contract bolt.  The first one did not pass, but the second one did. The person from J&T said he had only seen this happen a few times over many years.  By the way, J&T were great to work with.  

Dan