AR bolt hold open issue

[malinkyhoy]

Hello,


I’m having an issue with a rifle I built. I started life with a carbine buffer tube etc. I recently installed an A2 length buffer tube and the bolt will no longer remain open. Does not stay open after last round. Will not stay open if I try to hold open the bolt when empty, or with an empty magazine.

I installed another lower with a carbine tube and it works fine.

I replaced the bolt hold open on the lower with the problem and the issue persists.

Any suggestions?

 

[pell1203]

Given the above description... am I correctly interpreting that you are stating that when the A2 buffer tube is installed you are unable to lock the bolt back manually?

 

[malinkyhoy]

Yes, that is correct.

 

[pell1203]

Does the A2 buffer tube buffer and buffer spring all appear to install and align properly?

Is there any interference visible anywhere on the A2 assembly when compared to the carbine buffer?

 

[J McGriddle]

Did you also replace the buffer spring?

 

[flyer]

Weird problem, does the bolt hold open move freely?

Some times if you dry fire without catching the hammer on a bare lower it can bend the casting and pinch the bolt hold open slot.

That's about all I've got without getting my hands on it.

 

[Sniper266]

Pics would be great for this one.
I would try to functions check everything i can in the upper and lower.

-Bolt catch moves freely
-Magazines engage the bolt catch when empty
-bolt moves in carrier with easey “flick”
-carrier and bolt move easily though the upper
-with the rifle together, hold the bolt to the rear, depress the bolt catch, see if the bolt catch is high enough to engage to bolt.
-inspect the bolt lugs.
-double check to ensure you have the proper spring and buffer installed in the correct length tube.
-ensure the bolt fully cycles amd passes beyond the bolt catch.

If the problem was just the bolt not holding open after the last shot i would say that your rifle is under gassed or that your buffer wight or spring is too heavy causing the bolt to not cycle fully and pass the bolt catch

 

[Precision Underground]

WHat barrel and gas length is it? A2 is longer I believe and that means more travel. Bottom line is there is not enough gas to push the bolt back far enough to stay open. Maybe it’s the longer tube, maybe it’s a heavier spring or buffer. You need more gas or a softer spring or lighter buffer.

 

[malinkyhoy]

Pics would be great for this one.
I would try to functions check everything i can in the upper and lower.

-Bolt catch moves freely
-Magazines engage the bolt catch when empty
-bolt moves in carrier with easey “flick”
-carrier and bolt move easily though the upper
-with the rifle together, hold the bolt to the rear, depress the bolt catch, see if the bolt catch is high enough to engage to bolt.
-inspect the bolt lugs.
-double check to ensure you have the proper spring and buffer installed in the correct length tube.
-ensure the bolt fully cycles amd passes beyond the bolt catch.

If the problem was just the bolt not holding open after the last shot i would say that your rifle is under gassed or that your buffer wight or spring is too heavy causing the bolt to not cycle fully and pass the bolt catch

The buffer and spring are correct. I was wondering if the spring could be to long?

I can feel the carrier slipping off the catch when I try to arrest it. But I’ve replaced the bolt catch twice now with catches from GunWorks, PSA, and Anderson. The same upper also holds open fine when fitted to another lower.

Bolt cycles correctly.

I did think about short stroking but since it’s happrning when it’s unloaded I can’t think of anything else.

 

[malinkyhoy]

WHat barrel and gas length is it? A2 is longer I believe and that means more travel. Bottom line is there is not enough gas to push the bolt back far enough to stay open. Maybe it’s the longer tube, maybe it’s a heavier spring or buffer. You need more gas or a softer spring or lighter buffer.

Barrel is 18” Rainier Arms with a rifle length tube.

I thought about short stroking but it’s also not holding open when empty and charged manually.

 

[malinkyhoy]

Weird problem, does the bolt hold open move freely?

Some times if you dry fire without catching the hammer on a bare lower it can bend the casting and pinch the bolt hold open slot.

That's about all I've got without getting my hands on it.

It does and I’ve removed and replaced it twice.

 

[Precision Underground]

Barrel is 18” Rainier Arms with a rifle length tube.

I thought about short stroking but it’s also not holding open when empty and charged manually.

Ahh. I see. But it works fine with the carbine buffer tube?

 

[malinkyhoy]

Did you also replace the buffer spring?

Yes

 

[malinkyhoy]

Does the A2 buffer tube buffer and buffer spring all appear to install and align properly?

Is there any interference visible anywhere on the A2 assembly when compared to the carbine buffer?

No interference but I was wondering if the spring could somehow be to long.

 

[Precision Underground]

Have you tried a different empty mag and try to make it lock back manually?

Put an empty mag in and look into the upper while slowly letting the CH forward and see why it’s not catching.

 

[Precision Underground]

Look into the ejection port that is.

 

[Sniper266]

If the bold cycles back all the way, under the charging handle then your spring is not to long.

From everything I understand, this problem is not exclusive to when you fire and the bolt fails to lock back even when manually racked. This tells me that its not a gas issue.

Looking to see if the bolt is riding over the raised bolt catch i think is the key. If this happens at all, with the magazine in or your thumb on the bolt catch then thats where your problem is.

Looking forward to hearing the solution to this!

 

[Precision Underground]

Is this a small or large frame ar?

Buffer could be bottoming out in the tube. Do you have it screwed in a bit too far?

 

[EXTREMEPREJUDICE]

Buffer could be bottoming out in the tube.

Reading through the thread I thought of this also, almost like a rifle buffer in a carbine tube. Of course, since he has the A2 tube that isn't the issue but perhaps there is some buffer/spring/tube mismatch?

 

[natdscott]

Sprinkle parsley on it.

 

[malinkyhoy]

If the bold cycles back all the way, under the charging handle then your spring is not to long.

From everything I understand, this problem is not exclusive to when you fire and the bolt fails to lock back even when manually racked. This tells me that its not a gas issue.

Looking to see if the bolt is riding over the raised bolt catch i think is the key. If this happens at all, with the magazine in or your thumb on the bolt catch then thats where your problem is.

Looking forward to hearing the solution to this!

It was a school boy mistake. The buffer tube stock screw was to long. I cut some threads off and it now functions perfectly.

 

[flyer]

That's a new one. I'll have to remember that.

 

[FALex]

I hate to even have to suggest this, but have dealt with it on my very first AR: get a flashlight and look down into the base of the A2 tube. I had a very similar issue years ago and discovered that the individual who had the tube before me had stuffed a bunch of steel wool into the base of it. Don't ask me why the hell someone would do that, but it's no BS.

 

[natdscott]

I hate to even have to suggest this, but have dealt with it on my very first AR: get a flashlight and look down into the base of the A2 tube. I had a very similar issue years ago and discovered that the individual who had the tube before me had stuffed a bunch of steel wool into the base of it. Don't ask me why the hell someone would do that, but it's no BS.

Jesus..you could chase that issue FOREVER and never find that stupidity.

 

[malinkyhoy]

Here is the offending rifle. Should anyone be interest, the Aero Precision rail that I purchased on sale is okay. However the magpul mbus will not deploy with the rail attachment that is supplied with the rail. I'll be replacing the whole hand guard with something better made soon. The Nikon P mounts will be gone too.

 

[malinkyhoy]

Jesus..you could chase that issue FOREVER and never find that stupidity.

I bet he wanted to cheap out and use a carbine spring in it or possibly didn't know that an A2 spring existed.

 

[FALex]

I bet he wanted to cheap out and use a carbine spring in it or possibly didn't know that an A2 spring existed.

Yeah, I forgot to mention that I continued to find steel wool shavings each time I separated the receivers, though I did not connect the fact that these were steel wool shavings at the time. I would get some shavings in the FCG and on my bench. A friend surmised that some dipshit was engaged in a feeble attempt to mitigate recoil.

 

[Sniper266]

Glad to hear you figured it out! I dont know if i ever would have checked the screw for length.

 

[Elite_KG]

It was a school boy mistake. The buffer tube stock screw was to long. I cut some threads off and it now functions perfectly.

This and a bad magazine are the two most common reasons for the bolt not locking back I seen or heard about. The other would be from the gun being dirty. The first two are obviously mechanical issues whike the thrid is a maintenance issue.

I chased a bad magaine issue in my first pistol build. I swore it was a buffer weight issue since I was runnin the JP captured buffer. Changed the weights and springs a dozen times before a friend said to switch magazines. Instant fix. I could replicate the issue with bad magazine, but with several other mags it ran fine.