I'm just about finished on my first AR build, a Bare bones 20" A2. Its using a 20" A2 upper from colt, and a bolt carrier assemble I ordered from Mod.1. I know about the headspace issue from reading all of the posts, but *HOW* important is it to actually check it? Isn't all AR series chambers and blots made to the same spec.?

The Cheat

It's important enough that nobody in his right mind would dare fire a new rifle without having the headspace checked first, OK?

Get thee to a gunsmith and have him check it, or find someone with the gauges and read the directions!

It's important.  Really.  

Your rifle will PROBABLY headspace just fine, but if it doesn't, you're shooting a bomb with a stock attached to it.

CJ

Thanks CJ.

I'll have to bring the upper over to my local shop and have them check it out. If I doesn't space for whatever reason I assume the only "fix" is to swap out bolt heads? I can't see any other way to adjust it.

The Cheat

Correct.   You'd either have to swap bolts or barrel extensions, and you're not going to be swapping barrel extensions!

I can only imagine that the factories that make barrels and extensions must have buckets full of barrel extensions that they sort through to fit them correctly to individual barrels. If the threads on one extension or barrel are timed differently by any measurable amount, the extension won't clock right at its correct torque spec, and that's not going to work, as the index pin in the extension determines the barrel's position (very critical if the sight tower is already installed) and also this would change the feed ramp's position.

But then again, maybe the factories machine the lugs and locator pin holes AFTER the extension is torqued to the barrel.   Could be.  But when it comes to replacing an extension, only factories are going to be able to cut a new one in place.  

CJ

Just wondering if anybody's ever heard of an AR made of first-rate parts, that is, purchased from Bushmaster, for example, rather than something they got out of a bin at a gun show, ever being incorrectly headspaced? (The reason bin parts are cheaper is that they're often rejected seconds surplussed out by GI contractors.) May happen, but I've never actually seen it with good parts. And if this were a real concern, don't you think the big sellers of parts, again such as Bushmaster, would have a big fat warning on their website where they sell those parts? Not only for legal CYA, but also to sell you a headspace guage? Well, they don't...

These guns are made with such exact tolerances that I don't think it's a big problem. If the threads aren't indexed properly, the lugs on the barrel extension won't engage properly. And with modern production processes (CNC for example), the threads aren't just randomly cut, they are cut the same way every time with great precision.

The most likely departure from spec would be a bolt that has too much or not enough "room" for the cartridge. Again, with CNC manufacture, this isn't at all likely.

Having said that, of course, it certainly never hurts to check. As a precaution I try to make sure that my bolts and barrels come from the same maker, and have the headspace checked by a friend of mine who does such things for a living. But as I said, I've yet to see one that wasn't spot-on.

I have seen rifles assembled with junk parts kits that I would not fire with them tied to a tree, with a 50' string tied to the trigger, hiding behind a barrier.  You know those kits, the ones made from old M16's, imported from some third world country after they wore them out, who got them from Uncle Sugar after some National Guard wore them out, who got them from the Army after a gazillion new recruits in bootcamp wore them out.

But as far as new parts from reputable suppliers, no, I have not seen any problems with those, ever.