Armalite barrel + DPMS bolt headspaces ok, still safe?

[H_0_D]

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?

[jtallen83]

10 minutes ago, H_0_D said:

Is there any other reason?

Cause there different, at one time I saw a comparison drawing that showed it. There's a thread here somewhere that a bunch of us posted critical dimensions for all sorts of 308 AR bolts. In short just get an Armalite BCG and you can run the pair in what you want.

[Matt.Cross]

^^^ What he said. Use any carrier you like, but make sure your bolt type and barrel type are matched.

If you feel like gambling with your life or well-being, feel free to disregard this advice.

[H_0_D]

1 hour ago, jtallen83 said:

Cause there different, at one time I saw a comparison drawing that showed it. There's a thread here somewhere that a bunch of us posted critical dimensions for all sorts of 308 AR bolts. 

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.  

[jtallen83]

1 hour ago, H_0_D said:

But are there any other dimensions which are different,

Seems like the cam pin location or something measured from the cam pin location might have been different by a touch, don't recall for sure. I would recommend a bit of time on a search engine to find the drawings if you have to know all the details. I had the same question  years ago and wasted a good bit of time to find out the guy that said they weren't compatible was right. 

 Make sure to hit up the introduction section and tell us a bit about yourself, BEFORE you decide to try that passing headspace with live ammo.

[ARTrooper]

4 hours ago, H_0_D said:

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?

I had this problem with my .260 Remington build. Because a lot of manufactures through the term ar-10 around interchangeably so often, I got a barrel and a bcg that were both advertised for ar-10s. After it wouldn’t close on a no go gauge, I did a little research and found out my problem was ar-10 barrel to dpms style bcg. 
 

because I had custom ordered the barrel, I sent it back with my bolt and they fixed the head spacing issue. The thing is a little tight when trying to manually extract a round, but fires, extracts, ejects, and is really accurate. 
 

so if you have the option, instead of getting an armalite bcg, you can go the route of fixing your chamber and headspace. 

[98Z5V]

On 2/20/2021 at 1:22 PM, H_0_D said:

Interestingly, my brand new Faxon brand barrel,

The very title of your thread is "Armalite Barrel..."  

...and then you say you have a Faxon barrel.  Your Faxon barrel is NOT an Armalite AR-10-based or patterned barrel.

[Albroswift]

I think the OP was using the Faxxon as a comparison. You are allowed to have more then one barrel I think, in fact, may be required...

[H_0_D]

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.

[H_0_D]

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.   

[jtallen83]

8 hours ago, H_0_D said:

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.

Fire in the hole then, set up a camera and share the experience, definitive proof one way or the other I guess. 

For future information seekers that may stumble across this thread, pure sarcasm on my part, there is no good reason to ever mix the bolts and barrels on these platforms.

[DM15]

I too have been on the same trail as you.  I'm probably on many watchlists now with all the searching I've done.  I've about concluded that if 1) the headspace is in spec and 2) you overcome the difference in gas tube length, then it will work.  I think it is wise to not intentionally seek to mismatch components from the two different styles, but if that is what you have in todays climate then its worth investigating.  

I haven't found evidence that the bolt and extension lugs will damage each other, or any other damage to the rifle.   Once again it seems to be safety first and that is verified/denied by checking headspace.  

This is my opinion for now and not intended to guaranty success or safety for others.  

[Matt.Cross]

22 minutes ago, DM15 said:

I too have been on the same trail as you.  I'm probably on many watchlists now with all the searching I've done.  I've about concluded that if 1) the headspace is in spec and 2) you overcome the difference in gas tube length, then it will work.  I think it is wise to not intentionally seek to mismatch components from the two different styles, but if that is what you have in todays climate then its worth investigating.  

I haven't found evidence that the bolt and extension lugs will damage each other, or any other damage to the rifle.   Once again it seems to be safety first and that is verified/denied by checking headspace.  

This is my opinion for now and not intended to guaranty success or safety for others.  

Just go for it, if you lose a few fingers you can still dictate a nice post for someone else to type so we will know what the results of your experiment were. We're all about lessons learned and data points. Be sure to include pictures. You get bonus points if any part of your gun ends up embedded in any part of your head. Be warned, I WILL laugh and/or shrug and say you knew better.

This is your chance to be a pioneer and become a legend in your own time, don't let the naysayers get you down. 

[DM15]

No sir.  I certainly want to be safe and definitely don't want to encourage anyone else to try anything that might cause harm.  

Now please educate me as to where the problem lies assuming that HS is in spec.  If the bolt locks up properly, doesn't disengage to early, where will the danger lye?  Not arguing, just want to know what goes wrong.

[98Z5V]

14 minutes ago, DM15 said:

No sir.  I certainly want to be safe and definitely don't want to encourage anyone else to try anything that might cause harm.  

Now please educate me as to where the problem lies assuming that HS is in spec.  If the bolt locks up properly, doesn't disengage to early, where will the danger lye?  Not arguing, just want to know what goes wrong.

Nobody has ever been dumb enough to try it, so there isn't any "evidence" that you're going to find online.   If someone did try it - we've never heard from them again.

Have you found the thread that we have HERE on the measurements, and the differences between the two?  I'll link it in here if you haven't seen it yet.

Edited February 24, 2021 by 98Z5V

[98Z5V]

If you're than hung up on the Armalite barrel that you have, just buy a complete Armalite AR-10 BCG, and run it (the combo) in your DPMS-based receivers.

[Matt.Cross]

3 minutes ago, DM15 said:

No sir.  I certainly want to be safe and definitely don't want to encourage anyone else to try anything that might cause harm.  

Now please educate me as to where the problem lies assuming that HS is in spec.  If the bolt locks up properly, doesn't disengage to early, where will the danger lye?  Not arguing, just want to know what goes wrong.

The problem lies in what can be expected to go wrong when up to 62,000 PSI (Max 308 SAAMI spec chamber pressure) is applied to various components that were NOT designed or specified to be compatible with each other. There isn't a standard here, no uniform specification. It's not a small frame AR with a Mil-Spec shared among manufacturers. There is not only no guarantee of compatibility, there is no design nor specification of compatibility. I've worked in manufacturing and machining for quite a few years now and I know full well what tolerance stacking, complacency, and various other quirks of human nature and flaws within the human attention mechanism can mean to a finished product. Combine that with the random outcomes of mixing unmatched components from varying manufacturers with varying degrees of quality control and you're begging for a disaster. This surely can't be difficult to grasp.

[ARTrooper]

1 minute ago, 98Z5V said:

Nobody has ever been dumb enough to try it, so there isn't any "evidence" that you're going to find online. 

Correction, I am that guy dumb enough to try it. Lol. Although headspace on my barrel was fixed to allow my bolt to properly lock and chamber a round. 
 

before I had it fixed I knew that it not locking all the way wasn’t good... but not sure it would even shoot if I pulled the trigger, depends on where the bolt was in inside the carrier to allow the firing pin to travel far enough or not I guess. 
 

after I had the barrel fixed, haven’t had any problems since.

 

theoretically, if it wasn’t locked all the way and did shoot, it wouldn’t be to catastrophic. Pretty sure it would just be like a cook off in an open bolt machine gun. Been there, done that, got a bit of case shrapnel in my cheek, a broken extractor, and a ringing in my ears. 

[H_0_D]

11 minutes ago, 98Z5V said:

Have you found the thread that we have HERE on the measurements, and the differences between the two?  I'll link it in here if you haven't seen it yet.

Would you post the link?

[98Z5V]

 

[jtallen83]

10 hours ago, 98Z5V said:

Have you found the thread that we have HERE on the measurements, and the differences between the two?

Wish there was a way to get all that data into a pinned thread with a good title so it's easier to locate, lots of good data there.

[H_0_D]

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.  

Edited February 24, 2021 by H_0_D

[ARTrooper]

I feel like this thread has turned into measuring our caliper dimensions 🍌 and gone off track. 
 

either get an armalite bolt to work with the armalite barrel or see if you can’t get what you have to function and report back here on your findings by actually testing it. 
 

I don’t know why this thread has seemed to turned into a back and forth as it has.

 

[Matt.Cross]

3 hours ago, H_0_D said:

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.  

And likewise there's no definitive data that says that mixing components is safe, so for both reasons of common sense and forum liability, I'll continue to recommend folks match their bolts to their barrel type until you're willing to foot the bill for any lawsuits against the forum.

[98Z5V]

Run with scissors HOD - have a blast.