Ar pistol help 5.56

[BigOski]

Anyone have any insight on what I might be over looking. Built a 10.5 5.56 pistol.

Had failure eject and stove pipes randomly. With h/h2/H3 buffers I can get the firearm to fire and reset trigger and eject however the bolt won’t lock back when mag is empty nor pick up next round. Currently installed carbine buffer spring and adjustable gas block. Wide open gas block with all buffers mentioned above no lock back on empty or pick up next cartridge.

I removed variables such as folding adapter. Only thing I was thinking was the pistol buffer tube is tighter clearance not allowing a normal cycled bcg to lock back. Maybe a carbine spring isn’t the correct spring for ar pistol buffer tubes.

Any help would be appreciated

Thank
Mike


Sent from my iPhone using Tapatalk

 

[Little Jack]

Other stuff
Standard buffer?
How many coils/ length of the spring?
What Ammo?
Smooth when hand cycling?
Gas Block aligned?
How's the extractor/ ejector?
When it does eject, where's the brass go and how far?

 

[MadMac22]

Other stuff
Standard buffer?
How many coils/ length of the spring?
What Ammo?
Smooth when hand cycling?
Gas Block aligned?
How's the extractor/ ejector?
When it does eject, where's the brass go and how far?

Great questions

Most of my gas problems have come from Gas Block misalignment

When it does eject, use this chart to help

 

[BigOski]

Great questions

Most of my gas problems have come from Gas Block misalignment

When it does eject, use this chart to help

View attachment 137926

Factory Winchester and brownells 55gr

Almost new NIB bcg from a rifle build that functioned well. 2-3 o’clock ejection when I get it (adjusting gas block)
Aligned gas block.

Wide open gas block with h2 or h3 buffer - no lock back on empty mag but when I pull charging handle I can get it to “travel full length” and lock back.

Only issue I just discovered is the installed kak shockwave tube is 2mm shorter than a other pistol tube I have sitting in the bench. Not sure if that 2mm makes a huge difference and I swapped the tubes out already to the longer one. I just don’t seem to be getting the full travel of the bcg now when before I thought bcg was traveling too fast so I slowed it with adjustable and heavy buffer.

Thanks for the help and interest


Sent from my iPhone using Tapatalk

 

[Daezee]

Check gas key to make sure it’s tight and not plugged up. Check gas rings on bolt.

 

[BigOski]

Check those and good. Thanks for interest.

*** I have two twin builds with this issue.

One with new parts and the one I am discussing with known good parts. Both had folding stock adapter but this one with known good parts I removed and just swapped the kak shockwave tube off of to the 2mm longer one which matches other know pistol tube inner lengths


Sent from my iPhone using Tapatalk

 

[Daezee]

Measure length of buffer spring and post it here. Meet someone at a range and swap parts one at a time. Does the bcg slide easily within the upper? I once had an out of spec upper that would grab the bcg at random. Does your lower function with another upper assembly?

 

[BigOski]

10-9/16
10-3/4

Tried both of those and also tried my buddies sound mitigation buffer which is different but works in his pistol just not mine. No grabbing. Nice and smooth movement on pulls to charge


Just left Styx today doing just that with part swapping. I think that tube might be it with that 2mm shorter one but not sure cause that’s all I can see differs

Sent from my iPhone using Tapatalk

 

[Daezee]

Spring lengths are within specs. Does your upper just happen to be made by Black Sheep? A bunch of theirs were put on fire sale prices as they closed business and got out of the AR upper making business were not within spec. I know, I bought one...at random failures to eject, feed, etc, as the bcg would randomly stick as it moved under pressure of being fired. By hand it felt smooth.

Take a complete working upper assembly off someone else's gun and try it on your lower. Take your complete upper assembly and try it on someone else's lower. You need to start narrowing down it is something about your upper or your lower...it's just frustration at this point for you.

You say you have 2 builds with this same problem. Are both of your receivers (upper and lower) from the same maker? When you remove your gas block is the bbl gas hole within the "stain" left from the powder gasses?

Regarding plugged gas key: what can happen is a loose primer or other piece of metal gets into the gas key and the gas tube pushes it all the way back. At a glance, all seems OK, but running a pipe cleaner or a piece of wire into the gas key reveals a partial to complete blockage.

Not previously mentioned, but try different mags, especially of different makes to make sure nothing about the magazine is dragging on the bcg.

Too bad you're not closer to come over and try different uppers and lowers...I have uppers of short to long.

 

[BigOski]

Appreciate the info. I will try the pipe cleaner in the gas key and yes the gas block stain is aligned correctly.

The twin builds - same upper (different bcgs 1 new 1 known good ). Different brand lowers but both have that kak shockwave tube and blade which measure 2mm less room in the tube length.

I will have to try a buddies gun to narrow down the tube or the upper whenever he gets around to it. I think next range trip is gonna be with all my different buffer weights with the other pistol buffer tube (that has the 2mm that the kak doesn’t).

I must check out - my 556 rifle tube has the same inner tube length has the pistol one I am going to try (with the 2mm).

Thanks for the help this far.


Sent from my iPhone using Tapatalk

 

[CenturyMk3]

which mags are you running?

 

[BigOski]

Tried a few different pmags hex mags. And a carbon fiber one from the sept Tacpac


Sent from my iPhone using Tapatalk

 

[kidsoncoffee]

Is there any carbon build up around the gas block? This sounds like a misaligned block. It's undergassed and not cycling properly. You could put the lightest spring and buffer in there and if the block isn't aligned, it won't cycle. You can check by using a piece of wire with a small 90 degree bend on it and push it down the barrel into the block. If there's any blockage the wire will find it and tell you. Also try a set of calipers and measure off your dimple on the underside unless it's pinned. Make sure the measurements line up correctly. Also try moving the BCG by hand through the upper. If there's any resistance the upper could be bent. I've seen people bend their uppers by installing the barrel nut without the BCG in the upper and it flexing in on itself causing restrictions.

 

[Oldergeek]

Tried a few different pmags hex mags. And a carbon fiber one from the sept Tacpac


Sent from my iPhone using Tapatalk

Not to be a smart ass, but are you sure that you are adjusting the gas block in the right direction? You say it is wide open. I doesn't sound like it is. I managed to get into this exact situation one time when installing a new gas block on an old build.

 

[BigOski]

I am going to break down everything and start from scratch. Check the gas path from the barrel through everything to the bcg. I appreciate everyone input I will update post if I find anything and or what happened to fix it later on.

Have a safe holiday


Sent from my iPhone using Tapatalk

 

[lil'skeet]

I PM'ed you BigOski

 

[Murfpcola]

Here is a pic of a used KAK tube that I bought a while back. Another thing I did not see mentioned is the barrel manufacturer and gas

port size. I would assume manufacturers would have gas port size figured out by now but does not hurt to check

 

[Daezee]

If you are still waiting on your buddy to try his upper on your lower and vise a versa, you can always bring it to my house (5 miles north of Whiting Field) and we can test on different uppers and lowers that I know work, shooting it right in my yard.

 

[BigOski]

I appreciate it. Currently in tx but return soon to comb over everything for anything obvious then going from there. I will let you know


Sent from my iPhone using Tapatalk

 

[FNHman]

Sounds like not enough gas to push the bolt back. Pull your gas block and measure your gas hole, some barrels are made for special applications and come with too small holes for uncanned shooting. Also, some barrels have poor quality control and the hole is slightly off center... if thats the case then your gas block needs to rotate to match the holes up perfectly. If all looks good, measure the distance from your barrels gas hole to the step on the barrel to make sure its the same distance as your gas blocks. I had one block one time that needed pulled slightly forward to match up.