H_0_D

I understand the dogma of cautioning everyone to headspace and verify every build. It just seems that the caution is getting limited to mixing Armalite with anything DPMS gen 1, when the caution should actually be a broader warning of measuring and verifying every build even within the nominal Gen 1 spec parts. There is as much dimensional variation within same-pattern parts as there is across the patterns. I mean, even headspace gauges don't agree. And they are the thing we rely on to tell us if the other parts are okay. I guess there's no one to guard the guards.

The armalite barrel measured as within a 1.634" Clymer No-Go gauge with all the DPMS-spec bolts I have. That is the combination in question, and it worked safely. I also have a Faxon barrel, of DPMS-spec. It failed headspace with that gauge. I called them, and was told they use a Pacific Tool specification, which is 1.635" for the No-Go. The barrel seems to be right at that larger measurement. And that particular barrel has also been fired safely. Of course, both specs would be safely well within a Field gauge.

I rediscovered the old Armalite series of technical notes, which I remember reading about 20 years ago. It's great that they bothered to write them, and that this website maintains them. Note #6 has a detailed comparison of the AR10 with the SR25. It refers to the two bolt carrier assemblies as "virtually identical" and "interchangeable." It doesn't explicitly tell us to just mix and match them willy-nilly. But, given the level of detail provided about many contrasting aspects, there is a conspicuous absence of any mentions about obvious reasons for incompatibility. Unrelated, I did find mentions that some combinations of BCG parts, and especially omitting the Armalite's firing pin spring, may create an arrangement where the firing pin can protrude before the bolt is locked in battery. That effectively eliminates a huge safety aspect of the Stoner design, which is that the firing pin can't extend until the carrier is forward enough for the bolt to have rotated the locking lugs into engagement. Possibly that explains some of the (noticeably few) online reference to AR 308 kabooms, some of which seem to be from out-of-battery discharges. And yes, I took the build out to the sand pit with a 25 foot long string. It worked just fine.

https://forum.308ar.com/topic/11427-armalite-tech-notes-1/

Yup, I'd seen that thread. It has some good data points. Mostly it illustrates the huge dimensional variation in components which are all ostensibly DPMS spec. Most of the Armalite measurements fall within the statistical noise of those DPMS measurements; if anything, it's more a demonstration of the similarity of the two patterns, rather than the difference. Also, most cheapo dial calipers are only accurate to 0.002", and those measurements were compiled by multiple people using multiple different calipers, so we need to be mindful of that imprecision. The gas keys are the same part number across the AR15 and both patterns of .308. The relative position of the gas key forward or back has relevance for gas tube length, but wouldn't play any role in the bolt/carrier/extension mechanical locking. The width of the keys would all be the same spec, so they all function the same way as an alignment "key" within the upper receiver's guide slot. So I don't see how a slight forward or backward variation would affect safe functioning. The slight difference in the carriers' cam slot position doesn't necessarily make them noncompatible with different barrel extensions. A difference in cam track length (front to back on the carrier) doesn't mean incompatibility; for instance, various aftermarket .223 AR bolt carriers have altered cam slot length. A difference in cam track rotation (total degrees of turn) might mean incompatibility, but both Armalite and DPMS patterns have bolts with the same number and width of locking lugs, so presumably the same number of degrees of turn for bolt lockup. (It may mean that a bolt requires a matching position of cam pin hole to a certain carrier, but that's a different issue and would need to be directly measured.) The variations in the cam pin's rectangular head width might have an effect of its "keyway" effect of aligning the bolt in the unlocked position within the carrier and relative to the upper receiver slot. So these are all good data points, but we still haven't found any definitive info that mixing components is automatically unsafe for any reason other than headspacing variations.

Would you post the link?

I think I've worn out Google in all the searching, but I still haven't found any actual measurements or info (on any forum which), which actually show mixing Armalite and DPMS barrels and bolts to be categorically unsafe in every case. The measurements of bolt lug length, counterbore depth, extension to chamber spacing, etc. all reinforce that there is no guarantee of safe headspace when mixing parts. But I haven't found any definite info that shows a combination is unsafe if the headspace is actually measured and happens to be within spec.

The mention of the Faxon barrel was to illustrate that cross-pattern assemblies can have safe headspacing, while a bolt and barrel both nominally DPMS spec can fail headspace.

I've also been researching the cross-pattern compatibility, and have found mostly contradictory and demonstrably inaccurate info. Not all barrels that use the longer-than-.223 length gas tubes are Armalite. For instance the Brownells retro 308 barrels are DPMS pattern yet use the longer gas tube. So it's not certain that your barrel is Armalite brand. Also there seems to be no actual dimensional difference in the barrel extensions. I think the difference is in how deeply the chamber is cut into the barrel, which impacts the headspace measurement. My inference is that "barrel extension" became a shorthand way of saying a barrel with slightly longer or shorter headspace. If your build has already been fired and doesn't exhibit anything unusual, you're probably fine. But it would still be prudent to buy a $30 No-Go headspace gauge.

I've found many threads that measured differences in bolt lug length and in bolt face counterbore depth. Variations in those dimensions would affect headspace, so I understand the universal warning of not mixing parts and assuming headspace is okay. But are there any other dimensions which are different, and would make the combo noncompatible for another reason than headspace measurement? Interestingly, my brand new Faxon brand barrel, which is supposedly Gen 1 specs, fails headspace with every one of my 4 bolt heads.

I have an Armalite barrel that I would like to use in a build that is otherwise all DPMS gen 1 parts. I have four bolt heads (a Toolcraft, a Brownells, and 2x late Bushmaster / DPMS) of the gen 1 pattern. All of the bolts will not close on a 308 No Go gauge. So if a mismatched barrel and bolt combination has safe headspace, is it fully safe to shoot? There are many posts warning about not crossing the two patterns, but the warnings are all about the potential for unsafe headspace. Is there any other reason?