I may be answering my own question, but is dropping a bolt group into an upper fairly plug-n-play, or does headspace need to be checked each time?  Come Monday I'll be putting the bolt group of my post-ban bushy into a pre-ban M4 upper (that previosly had no bolt/carrier) and marrying that with the post-ban lower.  All parts are Bushmaster, BTW.

Well, improper headspace can lead to KBs, but the chances of Bushie bolts and Bushie barrel extensions  together having improper headspace is minute.  However, a pair of HS gauges (go and no-go) from Brownells costs under $50, so I would recommend checking it.  Hell, we spend hundreds and hundreds of dollars on gun stuff anyways, what's another $50; plus, they are good for a long time.  

PACorps,

I'll agree that the best bet is to use a M16 5.56 field gage, if you can find a real one although I believe William Ricca advertises them occasionally in the SGN. Chuck, You are correct in that this in the only gage for checking headspace on M16's in use by the military. But, you will not find one in a unit arms room.

Unit arms rooms are at the Organizational Level of maint and run by "Armorers" (unit supply folks), which basically means they are only responsible to ensure their weapons are clean, replace minor parts like handguards, extractors, buffers spring, etc. and that they're all there (inventory them). Though they do keep track of when the Annual Gaging was last done and are responsible for scheduling and submitting work request for the next gaging with...... Direct Support (DS).

The folks at the DS shop are "Small Arms Repairmen" that have the training and experience necessary to correctly inspect a rifle for serviceability using ALL the gages, which in turn have been calibrated to ensure they're accurate. Seems like I'm splitting hairs I know, but there is a distinct difference in the capabilities and know-how between these levels of maintenance. While working most of my career in the latter, I've seen to many F$%k-ups happen in the former to have much faith in most of them, but I digress.

In my experience, it is not at all uncommon to see rifles that will "eat" a Field gage. This is due to wear in the chamber/barrel collar, the bolt, or a combination of both. In any case it places the weapon in a Deadlined (unsafe to fire) condition that requires replacement of components and gaging to verify that it is safe fire before it can be issued. Weapons fired with varying degrees of excessive headspace will progressively suffer from any to all of the following:
1. Increase in malfunctions, FTEx, FTEj or short stroking and the feeding problems that come with it due the case stretching/swelling beyond it's normal limits and the associated strain on the operating system.
2. Split or ruptured cases, these usually result in the shooter getting a face full of gas, powder residue, etc. via the charging handle. I've seen more than one be evaced off a range to get their eyes flushed out.
3. Cracked and/or sheared off bolt lugs. By this point the weapon has usually had a history of malfunctioning and has hopefully not been issued, but sometimes all's it takes is one more round to make it "let go".
I have also witnessed a few KB's in my day and a couple of them could not be attributed to the usual suspects of stuck projectiles (squib loads), bore obstructions (dirt, mud, cleaning rod sections, etc.) or missing cam pins, which may likely have been the result of excessive headspace. Though determining so afterwards through gaging is difficult when components are in pieces. Most of the extreme examples I've given are usually found in units with old well used weapons or those that simply ignore routine and scheduled maintenance.

Not trying to scare anyone, just trying to prove a point that checking headspace IS important. The price of gages or taking it to someone who has them is a small price to pay for safety. And while I've given examples of Military weapons, many of which have seen more use than any civilain rifle ever will, I've also seen a LOT of commercial parts that have failed with a lot less use. In fact, the only bolts I've ever seen broken at the cam pin hole that were not the result of a KB have been commercial, some with relatively low round counts. Though I will admit the odds are that if you buy NEW components from known REPUTABLE dealers or manufactures, you will be O.K. 99% of the time. But is it worth the risk?

Wpns Man.

Another thing about headspace... it is influenced by the AMMO.  The chamber and bolt are just a container for the ammunition.  What if you only neck resize?  What if you full length resize?  There is more variation there than is to be found on 99.99% of most AR-15 bolt/ barrel combinations.

Bad brass (even new), or brass that has been reloaded too many times, wrong powder, etc, is certainly a much greater cause of kabooms than headspace.

Did you assemble your AR from used parts of unknown heritage?  There used to be cheap kits, actually disassembled M16's reimported from 3rd world countries.  Now why would those countries get rid of M16's?  They were worn out.. discarded.  So why would you want those parts?  If this is how you got your parts, then yes, you had better check headspace.  Even if it checks OK, please don't fire this rifle near me.

But did you buy new parts from Bushmaster, Colt, Armalite, J&T, Olympic, RRA, DPMS, M&A, Model 1, or any of a number of other reputable suppliers?   Then I HIGHLY doubt you will find a headspace problem -- even if the barrel and bolt were bought from two different suppliers.

A reputable builder on this board stated recently that in 20 years of assembling AR15's (and M16's) he has yet to find a bolt and barrel that did not headspace correctly.

Here is a practical guide:  Does commercial ammunition fit?  Does it chamber and extract smoothly?  Then there is sufficient headspace.

Go shoot.  Examine brass.  Are there loose or flattened primers?  Signs of leakage around the primers?  Signs of stretching just above the base (the web) shown by shiny rings around the circumference?

No? Good.  Enjoy your rifle.

Chuck,

No problem, it's a common misconception that the unit (Org) armorer has the tools to fix anything. As usually the unit armorer is the last guy the user informs of a problem at turn-in and then it's miraculously fixed and functions perfectly the next time the armorer issues it to you. Problem is though that he didn't tell you he took it to DS to have fixed, as that would ruin his image. Anytime your in Virginia your welcome to come over to our side of the parking lot.

Tweak,

Good links to the troubleshooting pages. Just so some tuning in do not misconstrue my previous post. The FTEx, FTEj and short stoking malfunctions I addressed should not mean that I normally attribute these malfunctions to excessive headspace. Rather, after diagnosing these for the usual causes and coming up empty sometimes the cause becomes evident after gaging.

A_Free_Man,

Your exactly correct in that one of the biggest culprits for troublesome guns can usually be traced to the owner having bought worn out surplus or out of spec parts kits to build their rifles with. Either because they were uniformed, being fooled by the low price or just because they're cheap. Whoever has been building for 20 years and never seen a rifle eat a field gage must have only worked with NEW GI parts or those from a 1st tier manufacturer. When dealing with Issue weapons that are used or even rebuilt ones, it is not uncommon on average to find 1 rifle out of say every 2 or 3 hundred that will eat a field gage. Sometimes more sometimes less depending on the age, use and maintenance pratices (or the lack therof) of a unit.

Wpns Man