Armalite barrel + DPMS bolt headspaces ok, still safe?

[Matt.Cross]

4 minutes ago, 98Z5V said:

Run with scissors HOD - have a blast.  

Darwin and Nike are both behind you: Go for it!

[H_0_D]

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

[H_0_D]

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.

Edited February 28, 2021 by H_0_D

[ARTrooper]

4 minutes ago, H_0_D said:

And yes, I took the build out to the sand pit with a 25 foot long string.  It worked just fine.

So did you have the headspace fixed though?

[392heminut]

2 minutes ago, ARTrooper said:

So did you have the headspace fixed though?

From his first post it seems the headspace was checked and found to be okay.

[H_0_D]

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.

[ARTrooper]

3 minutes ago, 392heminut said:

From his first post it seems the headspace was checked and found to be okay.

Maybe I am just dumb but I would have shot it by hand. Lol. Should have seen the things we shot in Afghanistan that were confiscated, now some of those were scary. 
 

I get “the forum” saying it isn’t an approved  and supported thing. Good thing I am just a member and not admin... I always say, “send it.” Then again I might be a little bit of a hill billy these days. 🤣

[H_0_D]

19 minutes ago, ARTrooper said:

I get “the forum” saying it isn’t an approved  and supported thing. Good thing I am just a member and not admin... I always say, “send it.” Then again I might be a little bit of a hill billy these days. 🤣

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.

Edited February 28, 2021 by H_0_D

[ARTrooper]

2 hours ago, H_0_D said:

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 thing you have to realize is people here are more than willing to take their own risks in some form or another, but they aren’t going to suggest someone else do something that could be risky. This is a place of learning safely about firearms and we pride ourselves in coming up with solutions to building functional firearms and sharing the facts. Nobody wants to create a bad reputation for the forum here.