I got the Bolt Carrier Lock-Back Blues

[Bart Noir]

Sounds like you should be drinking a beer when you hear my blues? Well, go right ahead. Have one for me also.

I just assembled a 14.5 inch AR with mid-length gas system. My gunsmith put on the gas block and the tube. The BCG is an NB-finished Fail Zero unit (full auto configuration) which works fine in another mid-length AR. The loading, firing, and ejecting goes without problem. But the empty magazines showed a problem with the BCG locking back.

I started with an H buffer and an extra strong Wolff recoil spring. The problem was that it almost never locked the BCG back on an empty mag. This happened with several flavors of Magpul, with a Troy, with a couple of different versions of Lancer magazines.

But the basic GI metal 20-round mag, with original design follower, always raised the bolt hold-open just right. It never failed to lock the bolt carrier back.

I started changing the weight of the buffer and the spring. The hold open problems continued but the Lancer mags started to do better. So now the gun has a normal weight carbine buffer and a normal strength carbine spring. The Lancer mags hold the BCG back most of the time, but not 100% of the time. The Troy and Magpul magazines still do not hold the BCG back.

And again, the basic GI metal 20-round mag, with original design follower, always raises the bolt hold-open just right. It never failes to lock the bolt carrier back.

Sometimes the failures let the BCG go all the way forward and sometimes it seems to hang up on the actual magazine follower. In other words I find the BCG just a little forward of the normal open position, and when the mag is removed the BCG slams forward. There are also scratches on the rear ramp of the highest part of the mag followers, except the GI one. The scratches look like a bolt lug is catching the follower so that this sometimes causes the BCG to stop forward motion above the empty magazine.

Any idea as to whether my BCG is moving too fast, or too slow? Think I can improve things by putting in a light-weight Seekins BCG? Should I be going back to my gunsmith and asking blunt questions about gas ports being misaligned?

Thanks for any and all thoughts on this.

Bart Noir

[SpeedRacer]

Here comes the obvious question...who made the barrel and what is the gas port size?

[Bart Noir]

I should have added that the barrel is a Daniel Defense with 5.56 chamber, and the problems happened with both 55- grain and 62-grain US-made 5.56 cartridges. Again, my only problem is with the nicely loose hold-open lever not getting up fast enough to hold the BCG back.

Bart Noir

[Jwknutson17]

Ive had one rifle act like yours over the years, and my problem was that the travel of the bolt catch would not travel far enough up on the bolt to keep it from going back into battery. The lower was out of spec. And it just barley caught the lip with only new mags that had enough tension to keep it from going into battery.

Another rifle with similar issues was the receiver extension was binding the BCG ever so slightly to slow the carrier down. This was an out of spec upper receiver that caused the issue. Receiver was replaced and issue went away. Same issue with a slightly bent receiver extension. But I was short stroking in the middle of mags.

Don't know if that at all helps you. Just few of my experiences. From the sounds of your DD barrel and there gas port size, and ammo are not the problem unless it was assembled incorrectly. Is your gas block a low pro or FSB? Taper pins, or set screws? Hard to miss-align a FSB with taper pins.

[smitty1157]

My last build had the same problem. It turned out to be the bolt hold open catch the little finger that is pushed up by the mag follower was to short. I installed a different one and did the same thing. I ended up installing a bolt hold open catch for a 9mm build, the finger is much longer so it can be shortened to the correct length to work with p-mags and any others. I never had any other problems.

[MistWolf]

Run a couple of hundred rounds through it before getting worried about it not locking back. Give the rifle a chance to break in and for the gas system to seal itself

[MegademiC]

Is the rifle well lubed? Do the pmags hold the bolt back when cycled by hand? If so, is it engaged well, or just barely hanging on? What brand lower and lpk do you have?

[The_Hammer_Man]

Is the rifle well lubed? Do the pmags hold the bolt back when cycled by hand? If so, is it engaged well, or just barely hanging on? What brand lower and lpk do you have?

Do all the the above mentioned things and check the offset between the gas block and the shoulder of the barrel near the gas port. It should be approximately 0.032 offset. Yes, I know a gunsmith assembled it but,, as a gunsmith myself I can say without any doubt that we are terribly human and do make mistakes at times.

[MistWolf]

Do all the the above mentioned things and check the offset between the gas block and the shoulder of the barrel near the gas port. It should be approximately 0.032 offset. Yes, I know a gunsmith assembled it but,, as a gunsmith myself I can say without any doubt that we are terribly human and do make mistakes at times.

That depends on the gas block and whether or not the gas port is located correctly. Some gas blocks are made to go all the way against the shoulder and sometime the barrel gas port is mis-located a little

[Bart Noir]

What brand lower and lpk do you have?

The upper and lower receivers are from San Tan Tactical, and the lower came with an ambi bolt release already mounted. It is nice and loose in its slot (unlike a certain Seekins ambi lower which I bought once) but I had not thought about a too short pin - stub- finger or whatever you call the part that catches the back of the magazine lifter.

I cannot figure out what an "lpk" is, so cannot tell you where it came from.

I think I will buy another GI hold-open unit, for comparison and maybe actual installation in this gun. I am not sure it will be correct for this gun since the lower has that ambi hold-open feature. Then again, it might. It will not cost much.

I'll examine the hold-open part as soon as I return from France, which will be in a few days. And while I believe that all the mags I used were nicely holding back the bolt during manual cycling, I can revisit that also after my return.

The DD barrel has a low profile gas block on it, which was made by Thunder Technologies LLC from somewhere near Portland, Oregon. It came with two set screws but I had my 'smith drill and pin the unit in place.

And advice to go shoot more is always welcome! Thanks for the replies. I had realized that I have no idea of the fast moving dynamics and relationships between the various parts, and changes caused to those dynamics by changing the weights of BCG and buffer, and the strength of the recoil spring. It is interesting that the replies do not address those items so I can see that I was way off base. Again, thanks.

Bart Noir

[Slatewiper]

The upper and lower receivers are from San Tan Tactical, and the lower came with an ambi bolt release already mounted. It is nice and loose in its slot (unlike a certain Seekins ambi lower which I bought once) but I had not thought about a too short pin - stub- finger or whatever you call the part that catches the back of the magazine lifter.

I cannot figure out what an "lpk" is, so cannot tell you where it came from.

I think I will buy another GI hold-open unit, for comparison and maybe actual installation in this gun. I am not sure it will be correct for this gun since the lower has that ambi hold-open feature. Then again, it might. It will not cost much.

I'll examine the hold-open part as soon as I return from France, which will be in a few days. And while I believe that all the mags I used were nicely holding back the bolt during manual cycling, I can revisit that also after my return.

The DD barrel has a low profile gas block on it, which was made by Thunder Technologies LLC from somewhere near Portland, Oregon. It came with two set screws but I had my 'smith drill and pin the unit in place.

And advice to go shoot more is always welcome! Thanks for the replies. I had realized that I have no idea of the fast moving dynamics and relationships between the various parts, and changes caused to those dynamics by changing the weights of BCG and buffer, and the strength of the recoil spring. It is interesting that the replies do not address those items so I can see that I was way off base. Again, thanks.

Bart Noir

LPK=Lower parts kit

[Benito]

Run a couple of hundred rounds through it before getting worried about it not locking back. Give the rifle a chance to break in and for the gas system to seal itself

Serious question here:
I understand a rifle breaking in, but have do not understand and have never heard of a gas system sealing itself. I would think that if there is a leak to begin with, it would either get worse, or at best stay the same. I cannot see how a leak would reduce over time.

[markm]

14.5 middy should run on an H buffer with a USGI spring. Did you get rid of the stupid wolf spring??

There's likely gas leaking somewhere. I don't know what DD's port size is, but they usually trend to the larger end.

[Ryno12]

Serious question here:
I understand a rifle breaking in, but have do not understand and have never heard of a gas system sealing itself. I would think that if there is a leak to begin with, it would either get worse, or at best stay the same. I cannot see how a leak would reduce over time.

You have to crack an egg down the barrel first..

:sarcastic:

Seriously, carbon buildup will possibly seal up any small leaks.

[skp]

14.5 middy should run on an H buffer with a USGI spring. Did you get rid of the stupid wolf spring??

There's likely gas leaking somewhere. I don't know what DD's port size is, but they usually trend to the larger end.

I thought about this too. You (OP) only mentioned using the H buffer with the stronger spring. Did you try it with the standard spring?

[Jwknutson17]

From the OP quoted below...

Also DD is known for larger ports on their barrels. Explains why most of the replies dont address the barrel gas port or the spring and buffer weights as an issue. Seems something else is wrong here. Install, out of spec something, or leaking gas, etc etc.


Sounds like you should be drinking a beer when you hear my blues? Well, go right ahead. Have one for me also.

I just assembled a 14.5 inch AR with mid-length gas system. My gunsmith put on the gas block and the tube. The BCG is an NB-finished Fail Zero unit (full auto configuration) which works fine in another mid-length AR. The loading, firing, and ejecting goes without problem. But the empty magazines showed a problem with the BCG locking back.

I started with an H buffer and an extra strong Wolff recoil spring. The problem was that it almost never locked the BCG back on an empty mag. This happened with several flavors of Magpul, with a Troy, with a couple of different versions of Lancer magazines.

But the basic GI metal 20-round mag, with original design follower, always raised the bolt hold-open just right. It never failed to lock the bolt carrier back.

I started changing the weight of the buffer and the spring. The hold open problems continued but the Lancer mags started to do better. So now the gun has a normal weight carbine buffer and a normal strength carbine spring. The Lancer mags hold the BCG back most of the time, but not 100% of the time. The Troy and Magpul magazines still do not hold the BCG back.

And again, the basic GI metal 20-round mag, with original design follower, always raises the bolt hold-open just right. It never failes to lock the bolt carrier back.

Sometimes the failures let the BCG go all the way forward and sometimes it seems to hang up on the actual magazine follower. In other words I find the BCG just a little forward of the normal open position, and when the mag is removed the BCG slams forward. There are also scratches on the rear ramp of the highest part of the mag followers, except the GI one. The scratches look like a bolt lug is catching the follower so that this sometimes causes the BCG to stop forward motion above the empty magazine.

Any idea as to whether my BCG is moving too fast, or too slow? Think I can improve things by putting in a light-weight Seekins BCG? Should I be going back to my gunsmith and asking blunt questions about gas ports being misaligned?

Thanks for any and all thoughts on this.

Bart Noir

[skp]

From the OP..

Also DD is known for larger ports on their barrels. Explains why most of the replies dont address the barrel gas port or the spring and buffer weights as an issue. Seems something else is wrong here. Install, out of spec something, or leaking gas, etc etc.

Yeah, he mentioned trying an H buffer with an extra strong spring, then switching to a standard spring and buffer. He did not mention trying any other combinations.

[Jwknutson17]

Yeah, he mentioned trying an H buffer with an extra strong spring, then switching to a standard spring and buffer. He did not mention trying any other combinations.

I hope he tried the H buffer with Carbine spring. That is a well known performer for that set up. I run H2s in my middy's. Even H1's with Tubbs flat wire springs still run in DD mids with quality 5.56 ammo. So I'm having a hard time believing that the spring and buffer is the cause of the issue. That's my opinion based on my personal experience of course.

[CoryCop25]

My guess (with limited knowledge of the set up) is a misaligned or out of spec gas block.

[markm]

The barrel likely has an .080" port at a minimum... which provide plenty of gas for the configuration. I've had a 14.5 middy get weak with an out of spec/worn gas tube that let gas leak out the front of the key. There could also be a leak under the key.

[Chunky_Lover]

something easy to check , as it happened to me on a pistol

measure the inside depth of your buffer tube
I think you want 7-1/4 inches deep
if you have another rifle handy check that one and see what it measures too

the one I had on the pistol did not lock back the bolt empty mag, buffer showed signs of dings on the plastic endcap which I believe was caused by hitting the inside of the tube as it wasnt long enough

the buffer tube was 1/4 inch shorter then 2 other ones I checked
once replaced locks back the bolt as it travels back a touch more to allow this
but at that same time I had put in the next heavier buffer but im pretty sure it was the tube length


now if I had left it maybe shooting a thousand rounds would of deformed the plastic cap on the buffer enough to lock back but as it was the buffer was some china brand and started to unscrew im sure it would of just broke apart so thats why I replaced it at the same time putting on new tube

[Iraqgunz]

Something tells me that the issue involves the words "gunsmith" "Thunderball gas block" and "pinning in place".

[Bart Noir]

To those of you who mentioned that the bolt catch might be the problem, I say, 'well done'. Once I opened the receivers and looked at the different magazines interacting with the bolt catch that came from San Tan, I could see that the arm on the front was too short to let the mag follower reliably push the bolt catch up.

Then I left town for many weeks and didn't work on it. But now there is a new bolt catch in the lower and it has no problem with being lifted by any magazine.

Thanks for the responses.

Bart Noir