HELP! - 9mm AR build and headspace questions

[tripletaco]

Im putting together a 9mm AR upper but Im not sure if this is quite right. If I understand correctly in a 9mm blowback system the case should be sandwiched between the bolt and barrel, with the case head contacting the bolt and the case mouth resting on the end of the chamber.

What I have, is the bolt face resting directly on the barrel face. When a cartridge is in the chamber I can use a small pick and move it back and forth. Im not using the best caliper but I measure the depth of pocket where the case head rests at .135-.138". Using a empty 9mm case with length of .744, I measure about .115-.118 extending past the face of the barrel. The barrel also has a slight countersink around the chamber. I cant measure precisely but i think about .135" of case is exposed from the base of the countersink.

Basically what Im getting at is when a cartridge is in the camber and Bolt closed, there is about .020" of clearance where the cartridge can move back and forth. Is that correct?

http://www.northeastshooters.com/vbulletin/threads/279504-This-is-why-you-check-headspace-KABOOM

My reading says you typically would want no more than .100-.135 of case protruding from the chamber.....which is what I have. That leads me to think my barrel is ok and my bolt is cut too deep. Either because its out of tolerance or its just that this bolt/barrel are not compatible.

FYI the bolt is a Faxon firearms and the barrel is a Ballistic Advantage

thanks!

 

[ruebarb]

I would recommend getting a go/no go gauge to verify the head space

http://www.brownells.com/gunsmith-t...space-gauge-kit-sku184000042-26876-52458.aspx

 

[Green_Manelishi]

I am curious to know "why?" are you doing such a thing? It seems an overly large platform for a 9mm round.

 

[tripletaco]

I would recommend getting a go/no go gauge to verify the head space

http://www.brownells.com/gunsmith-t...space-gauge-kit-sku184000042-26876-52458.aspx

Verifying the headspace is all well and good, but I think my hang up lies in the theory of doing that.

The bolt in a blowback 9mm doesnt have a defined "lockup" position like with a typical rifle cartridge. The cartridge headspaces off the case mouth ( I think). Depending on the depth of the camber and the depth of the pocket cut in the bolt face determines where the "in battery" position of the bolt is. The bolt face may seat against the barrel face, in which case there is probably clearance between the bolt and cartridge case. Thats what i have here. The other situation is the bolt seats on the case head without touching the barrel face. In that situation the case is held in compression between the bolt and the end of the chamber.

I guess my question is....Is a clearance situation ok for a blowback system?

 

[tripletaco]

I am curious to know "why?" are you doing such a thing? It seems an overly large platform for a 9mm round.

I'll address this briefly. This thread is to collect some information on how a blowback system should be setup, not to discuss the merits of rifles chambered for a pistol caliber. Id prefer to keep it on topic.

I dont think anyone needs much more of a reason than "because its fun" but there are others too. Id' agree in the full 16" barrel rifle configuration some of the merits are diminished. Not every gun in everyone's safe needs to have a defined tactical mission. Its something new and different to learn about. I enjoy collection parts, and assembling things. Depending on the situation it could be useful on shorter ranges, or ranges where only pistol calibers are allowed. Cost of ammo is about 30%+ less with 9mm than 5.56. The 9mm AR lowers often use glock magazine. Many people like the compatibility, if you already have glock pistols. You have the option with 9mm to use much shorter barrels which makes for great SBR setups. You also have the option with 9mm to use subsonic ammo which makes for a much better suppressed setup (if you can posses that type of stuff).

 

[Green_Manelishi]

Thanks.

 

[SJan]

I see that your bolt uses an AR style extractor, I'm not familiar with that specific bolt, is it possible that it is to be used with a matching barrel from the same manufacturer?

Here are my barrel and bolt, both CMMG, excuse the dirtyness, I shoot this thing, alot.

It appears that your bolt face is deeper recess than mine.

 

[dixiejarhead]

With blowback as long as the case and bolt are completely seated it should be just fine. I have a few different builds that seat the same and have not had any issues from them. Check fired casings for swelling at base. If so then yeah you have an issue. If not, don't worry about it.

 

[TomMontana]

I used a snap cap so I can see the red case body. In my BA and New Frontier barrels, I can see a smidge of red with the round in the chamber. In my Faxon, I can see a bit more red, similar to OP. The Faxon bolt face in the OP appears to have a deeper recess than my CMMG and NFA bolts.

Gun porn below: CMMG on left. NFA on right. My cheap calipers indicate ~0.12"/3mm depth. Not sure what it all means based on everyone's different builds. Just happy nothing bad happened... yet.

 

[tripletaco]

Update....

Thanks for all the responses. It seems I was probably WAY over thinking this. I took the gun out this past weekend and it ran perfectly. I was also told by a reliable source not to worry if the cartridge has clearance in the chamber to breech face because it will "headspace" off the extractor.

I did learn a couple things though. It seems bolts with the external AR type extractor such as the Faxon and QC10 have the bolt recess cut at .135" . Apparently faxon cuts their barrels so that nominal case exposed from the chamber is also at about .135" . As best I can tell many of the standard style 9mm bolts such as the CMMG, NFA ect... have the bolt recess cut around .120" deep. Im not really sure what is the norm with other barrel manufacturers. My Ballistics advantage has .115" of exposed case, as measured with a .744" case. SAAMI case length tolerance is .744-.754" .

 

[SJan]

9mm is a tapered case. It head spaces off of the case mouth. Not off of the rim like a 38 special or 357 Magnum. Sure the gun will fire with the extractor holding the case in place. A problem in some 9mm carbine barrels is some bullet shapes were touching the rifling before the case being fully seated in the chamber. A well-made barrel will have the lead from rifling to chamber cut a little longer to prevent this problem. Some manufacturers are taking a cheaper way out and simply running the chamber reamer deeper. This is not the same. You may notice your ejected cases look a little sooty, because they are not fitting tight in the chamber. This can result in decreased accuracy, but the biggest thing is the chamber will get filthy quickly and cause failures.

I don't know who you are reliable source is, but I know a thing or two about pistol caliber carbines. Hint look at number 1 GM in PCC division.https://www.uspsa.org/top20.php

 

[tripletaco]

Ok, Im listening. Are you saying the bolt should be firmly pressing on the case so that the case mouth is seated at the end of the chamber? I dont know that mine is truly head spacing off the extractor. The case has .020" of "play". Maybe the firing pin is pushing the case forward, to seat the mouth against the end of the chamber and striking the primer all in one move. Would that be an undesirable scenario?

 

[SJan]

Every part is designed with an ideal measurement, but there are acceptable tolerances in the manufacturing process. If a barrel chamber is on the deep end of the spectrum of the allowable tolerance, and a bolt face is also on the deep end of the allowable tolerance, the combined 'headspace tolerance' is going to be sloppy or loose, for lack of better description. If the extractor is holding the case against the bolt face, and there is excessive chamber depth that the extractor is preventing the case from moving forward into, a case rupture is very possible. Case ruptures are somewhat common in even the most well fit blowback firearm, simply do to the nature of its operation. The bolt is opening while there is still tremendious chamber pressure. There is much less pressure involved in a 9mm compared to a rifle round such as 223/556, but they have a rotating locking bolt.

The case has .020" of "play". Maybe the firing pin is pushing the case forward, to seat the mouth against the end of the chamber and striking the primer all in one move. Would that be an undesirable scenario?

That could result in light strikes, maybe, maybe not.

With the bolt closed and a round chambered: the case mouth should be seated in the chamber, the case head should be against the bolt face, and the bolt shroud should be against the barrel extension. There should not be a measurable amount of movement of a chambered cartridge.

 

[tripletaco]

If a barrel chamber is on the deep end of the spectrum of the allowable tolerance, and a bolt face is also on the deep end of the allowable tolerance, the combined 'headspace tolerance' is going to be sloppy or loose, for lack of better description.

OK right. So thats what I have. Relatively speaking, .020" of clearance is on the very loose side.

That could result in light strikes, maybe, maybe not.

None so far. The strike marks on the primers seem pretty solid to my untrained eye.

With the bolt closed and a round chambered: the case mouth should be seated in the chamber, the case head should be against the bolt face, and the bolt shroud should be against the barrel extension. There should not be a measurable amount of movement of a chambered cartridge.

Ok, so what your saying is my current setup is not good, or at least likely to have problems. Is it unsafe? There will always be some tolerance to deal with but things should be a lot closer than they currently are. Given that case lengths vary a bit. Do you want to set it up so that the bolt face and end of the chamber are firmly clamping the case. In which, there is probably a slight gap (couple thousandths) between the bolt shroud and barrel extension. OR....would you let the bolt seat against the barrel extension and have the headspace dimension just slightly larger than the case length?

Thanks for the help by the way. this is great info.

 

[stringpark]

OK right. So thats what I have. Relatively speaking, .020" of clearance is on the very loose side.



None so far. The strike marks on the primers seem pretty solid to my untrained eye.



Ok, so what your saying is my current setup is not good, or at least likely to have problems. Is it unsafe? There will always be some tolerance to deal with but things should be a lot closer than they currently are. Given that case lengths vary a bit. Do you want to set it up so that the bolt face and end of the chamber are firmly clamping the case. In which, there is probably a slight gap (couple thousandths) between the bolt shroud and barrel extension. OR....would you let the bolt seat against the barrel extension and have the headspace dimension just slightly larger than the case length?

Thanks for the help by the way. this is great info.

Sorry to bring up an old thread, but just wondering how things turned out for you on this @tripletaco? I have a similar situation, case protrudes .125” past barrel face and bolt depth is .138”, so there is .013” of “play”. Haven’t taken it to the range yet, so I don’t know if this is even a problem or not. Did the .020” of play end up being problematic for you?

 

[tripletaco]

Short answer...its not really a problem but its also not really am ideal situation. My gun works. I havent shot it in a while but I put 600 or 700 rnds through it with 1 or 2 malfunctions. Those 1 or 2 malfunctions were with federal aluminum cased ammo, so take that for what its worth.

What it comes down to is 9mm is a pretty low pressure round and blow back is a very forgiving operating system. It mostly just works. Its probably not the way its SUPPOSED to be though. What is happening with excessive head space is the case is allowed to move around. The extractor is holding back the rim enough that the firing pin can smash the primer well enough. All the points brought up bu SJan a probably valid. It probably opens you up to to the possibility of light primer strikes, ruptured/bulged cases or poor accuracy. I have not seen any of this...but again take that for what its worth.

 

[reloader41]

So reading thru all this, do you think that it makes sense on an AR9 build to stick with the same manufacturer for the bolt and barrel?

 

[teamRR]

Sorry to bring up an old thread, but just wondering how things turned out for you on this @tripletaco? I have a similar situation, case protrudes .125” past barrel face and bolt depth is .138”, so there is .013” of “play”. Haven’t taken it to the range yet, so I don’t know if this is even a problem or not. Did the .020” of play end up being problematic for you?

What brands are you working with?

I have headspace guages and was able to verify a BA barrel & Spikes, as well as a couple other brands of BCG, sit just a hair from touching the barrel extension with a "go" gauge, ie round is compressed tight. And that the bolt sits a bit more open on the "no go". Reasonably I am sure they are all using some standard headspace spec. The odd part of the equation w/ the OP's build seems to then be Faxon.

I am considering using a Faxon integral flash hider barrel on a future 9mm Colt style - basically so I can have a muzzle device at exactly a 16 inch barrel and to be able to run a normal stock. My problem, if anything, then will be although everything is going to be held tightly in place with the bolt closed, a touch more of the case is going to show if Faxon barrels have a shallower chamber designed around the deeper recess in their bolts and I use someone else's BCG.

I of course might be able to consider a Faxon BCG to be consistent, which is always the safest bet, to match a BCG and barrel manufacturer.

 

[creedbratt0n]

My eyes are wide reading this thread as someone (for the first time ever) building out a recently acquired AR9 lower and has already ordered all their upper parts.

 

[teamRR]

My eyes are wide reading this thread as someone (for the first time ever) building out a recently acquired AR9 lower and has already ordered all their upper parts.

The 3 problems I hit using spikes upper, lower, bcg and a BA barrel were:

Tight upper and lower fit - very minor filing and worked the upper and lower on/off resolved to a perfectly tight fit.

BCG wouldn't quite clear the ejector, basically upon closing the upper the BCG would be jammed in place. Contacted spikes who sent me a new ejector but basically said, this is a normal part of fitting the parts and to dremel a little material off as required. Easily resolved as they described.

Ejection problems - tried another spikes lower as well as other BCG's, other spring and buffer combos, and was consistent. In the end it took bending the ejector so it faces toward the ejection port a hair. This you will read about in researching.

Its good and reliable now, Colt version in my case and may or may not apply to Glock.

 

[creedbratt0n]

The 3 problems I hit using spikes upper, lower, bcg and a BA barrel were:

Tight upper and lower fit - very minor filing and worked the upper and lower on/off resolved to a perfectly tight fit.

BCG wouldn't quite clear the ejector, basically upon closing the upper the BCG would be jammed in place. Contacted spikes who sent me a new ejector but basically said, this is a normal part of fitting the parts and to dremel a little material off as required. Easily resolved as they described.

Ejection problems - tried another spikes lower as well as other BCG's, other spring and buffer combos, and was consistent. In the end it took bending the ejector so it faces toward the ejection port a hair. This you will read about in researching.

Its good and reliable now, Colt version in my case and may or may not apply to Glock.

I appreciate the tips, that’s great to know. My lower is a dirty bird with an ar9 magwell that excepts Glock magazines. Just waiting on my upper to come back from cerakote, hopefully the growing pains are minimal.