AR bolt wont lock open

[arnie19]

On my new 6.5 Grendel build ,every thing functions fine except the bolt doesn't lock back most of the time on the last shot. Locks back fine by hand. Load is the same that functions fine in my other two Grendels .Tried stretching the mag spring and different mags and still doesn't lock back .Im guessing removing some weight from the buffer might cure this ,or removing a coil or two from the buffer spring ??

 

[jasonfaz]

I'd check gas block alignment before clipping springs and trimming buffers. Any pics of the rig you can throw up?

 

[Redmanss]

My money is on over gassed, not under gassed, but how far the brass is being thrown on ejection will tell that tale better. Try a heavier buffer or close the gas block up some is you have an adjustable.

 

[jasonfaz]

Could be a bunch of things I guess. OP, what length buffer system are you running? Rifle, A5, carbine? Do you have an adj has block? Clamp on or set screw, etc? A little more detail might help pin point the issue...

 

[TheGerman]

H3 buffer

 

[bustin]

I'd check gas block alignment before clipping springs and trimming buffers. Any pics of the rig you can throw up?

^^This. If the brass is being tossed straight to the right or 4-5 O clock it is more than likely under-gassed or gas block mis aligned.
If the brass is being tossed forward of the ejection port it could be over-gassed and causing the carrier to bounce off the buffer but it would need to be WAY over-gassed.

 

[Captain Kirk]

With a empty mag in, pull the charge handle back and note how much room the bolt continues back after the bolt catch pops up. If the bolt doesn't go back far enough the bolt catch won't have time to engage, if this is the case just trim a little off of the buffer. It may function properly by hand, but the bolt needs to have about 1/4 clearance past the bolt stop for proper function. Second possibility is a gas issue.

 

[bigbear_98]

I guess we should first know what gas system, weight buffer, and type of buffer system you are using before we can help you.

I had tried to run a rifle spring and buffer in my Grendel with a rifle gas tube. I was having the same issues. I dropped a part that spikes tactical makes to fill the gap and installed a carbine spring and regular carbine buffer. Grendel runs like a sowing machine now. I would try a lighter buffer before you do anything since it is so easy to swap.

 

[MLC]

Did you build the lower?
When you say it locks back fine by hand do you mean on an empty magazine or by manually actuating the catch?
I ask because on my first build(in 1998) the bolt catch was binding from an out of spec or poorly installed roll pin.

 

[arnie19]

I have my camera charging right now and will try to get pictures later today .The lower is one of 8 I have built from 80% .The upper is a smooth side which I modified to a side charger .The bolt doesn't come back all the way to lock .We proved that by putting a piece of tape behind the bolt in the locked open position across the slot the handle moves in .We then fired it and the side charging handle doesn't touch the tape .If I pull it back by hand it locks open just fine and hits the tape . It locks back by the spring tension of the magazine and the bolt stop is free .I am going to try a lighter buffer and see if that cures the problem . I was using a brass catcher so not sure where the brass would land if free to fly out on the ground .BTW I was using PPU brass and they look perfect on the base .Not even a mark of any kind like they were shot in a bolt gun . Usually I see signs of the ejector swipes even if its slight but this one isnt doing that.

 

[bigbear_98]

What buffer do you have now?

 

[arnie19]

<a href="http://s227.photobucket.com/user/caarnie/media/argrendel004_zps5e2505f6.jpg.html" target="_blank"><img src="http://i227.photobucket.com/albums/dd62/caarnie/argrendel004_zps5e2505f6.jpg" border="0" alt=" photo argrendel004_zps5e2505f6.jpg"/></a>

 

[arnie19]

<a href="http://s227.photobucket.com/user/caarnie/media/argrendel003_zps641ed615.jpg.html" target="_blank"><img src="http://i227.photobucket.com/albums/dd62/caarnie/argrendel003_zps641ed615.jpg" border="0" alt=" photo argrendel003_zps641ed615.jpg"/></a>

<a href="http://s227.photobucket.com/user/caarnie/media/argrendel002_zpsa0d71488.jpg.html" target="_blank"><img src="http://i227.photobucket.com/albums/dd62/caarnie/argrendel002_zpsa0d71488.jpg" border="0" alt=" photo argrendel002_zpsa0d71488.jpg"/></a>

<a href="http://s227.photobucket.com/user/caarnie/media/argrendel001_zpsdd3944a0.jpg.html" target="_blank"><img src="http://i227.photobucket.com/albums/dd62/caarnie/argrendel001_zpsdd3944a0.jpg" border="0" alt=" photo argrendel001_zpsdd3944a0.jpg"/></a>

<a href="http://s227.photobucket.com/user/caarnie/media/argrendel007_zps19e3fe86.jpg.html" target="_blank"><img src="http://i227.photobucket.com/albums/dd62/caarnie/argrendel007_zps19e3fe86.jpg" border="0" alt=" photo argrendel007_zps19e3fe86.jpg"/></a>

<a href="http://s227.photobucket.com/user/caarnie/media/argrendel006_zps8f8693e4.jpg.html" target="_blank"><img src="http://i227.photobucket.com/albums/dd62/caarnie/argrendel006_zps8f8693e4.jpg" border="0" alt=" photo argrendel006_zps8f8693e4.jpg"/></a>

 

[bigbear_98]

Is there any writing on it. Might be a stamp on the font. H, H2, etc

I would bet money that if you took the side charge handle off, and shot it, it would lock back.

ETA: Dont get me wrong, everything you have there looks badass. You can make it work but I would try taking weight off first.

 

[arnie19]

I don't know the maker of the buffer or spring .Those two plus the tube and the stock came as a set and im not to sure where now as I have had them for almost a year .Could be Brownels ,Sarco or Numrich Arms . The spring has about 34 to 36 coils and the buffer weighs 4.5 ounces .I removed one weight and added a spacer .It now weighs 4.15 ounces and I will try it Saturday. As you can see by the pictures ,the bolt locks open fine by hand but doesn't come back far enough to lock when fired .We put tape on the upper so the bolt handle would hit it if it was coming back far enough and it didn't hit the tape so its just not cycling far enough back to lock but enough to eject the case .

 

[arnie19]

I will also try taking the side charger handle off .It might have added just enough weight to the carrier to be border line to heavy.

 

[bigbear_98]

Standard CAR 2.9 oz.
H buffer is 3.8 oz.
H2 buffer is 4.6 oz.
Rifle buffer is 5.17 oz.
9mm buffer is 5.5 oz.
H3 buffer is 5.6 oz.

http://heavybuffers.com/reference.html

This is what I found. I would suggest putting a CAR in there and trying it. Maybe from one of your other rifles?

 

[arnie19]

The buffer is probably the H2 since it weighed 4.5 ounces .If its still not quite rite Saturday ,I can remove one more weight .there easy to take apart . I have another side charger ,I will have to take the buffer out and weigh it .

 

[PopSmoke]

Gas block first. If you set up a video camera pointed at your ejection port, slight angle rearward to see the bolt going into the tube, you should see whether your bolt is going bak behind the magazine as it should. If not, you have a short stroke happening. DPMS had this notorious problem on the LR-308 platform prior to 2-4 years ago. Myself and a retired M16 armor did the same thing for the same symptoms to identify ours was a gas block problem resulting in a short-stroke of the BCG.

If the gas block is ruled out, change ammo. If you know someone close with the same caliber platform, do a side-by-side test. If the issue still remains start looking at other problem parts and get with those who have built your specific caliber.

 

[kyle528]

Sweet rig. Try locking the bolt back and let it sit for a few days to break in the spring before you go hacking away...

 

[bigbear_98]

Well what did you end up doing?

 

[arnie19]

I removed a 1/2 ounce and its locking back on the 100 gr. bullet but not every time with the 123 gr. bullet so I removed another 1/2 and will test again Tuesday . I also acquired a carbine buffer to work with .

 

[LRRPF52]

The most important details of this rifle are still missing.

What is the barrel and gas system length?

What is the gas port diameter?

What loads are you using? I see 100gr and 123gr bullets, but no indication of hand loads or factory, no specific data there.

Don't mess with another thing until that information is provided. Once we know the above, things will make a lot more sense.

 

[msalm]

I'd probably stop dicking around and just open the gas port .002-.005" and be done with it. Adjust the gas system so STANDARD parts work with it. Standard rifle length buffer, standard spring, etc. The side charging handle's mass more than likely gave you the issue, adjust the gas port accordingly and move on (of course double check gas block is properly aligned first as mentioned).

 

[sandwarrior]

FWIW, I had this same issue. What I found was, when putting the lower together, that two springs seem almost identical. They got swapped incorrectly. I swapped the correct spring into the lockback mechanism (the one under the lever on the LH side) and after that it worked fine. The one in there, IIRC, was softer than the one that is supposed to be there.