So I finally got a chance to take my nice new SBR out the range today. I was happy with the fit and finish of everything including the stock. However when it came time to shoot it I ran into a problem. I have an adjustable gas block on the rifle, I followed the instructions with setting it. I kept backing the adjustment screw out till I reached the maximum it could go. Yet every time I fired a round the rifle refused to have the BCG stay fully open on the last round. I used the normal procedure of firing one round and checking to see if the BCG stay open on the last round. With the gas adjustment at the max the best I could get is to have the bolt stay open but not far enough back to actually strip a new round into the magazine. So out of frustration I set the adjustment screw back to its closed position and opened it up just enough to let the gas in, then mounted my suppressor on the rifle. Low and behold, after opening the gas adjustment just a little more and with the suppressor mounted the rifle functioned flawlessly.

Granted I plan to use a suppressor on the rifle as often as I can, the fact that the rifle failed to function and only did function when the suppressor was mounted is frustrating. I mean I want to be able to use the rifle with or with out a suppressors, and not have to treat the rifle as a single shot bolt action. Any help or advice is greatly appreciated as this seems to be the failure point on the rifle.

Sounds like the gas port on the barrel needs to be opened up a little.

Gas block is most likely messed up.

There are VERY few barrels made today that are undergassed. Most are overgassed, hence the prevalence of bandaids like adjustable gas blocks.

The gas port is likely undersized for the VERT Stock System Buffer.  The internal secondary spring will require a bit more energy, compare to the standard Carbine Buffer system with a single spring, to cycle and lock back.  You need to compress the secondary spring of the Buffer by about 1/4"+ to allow the BCG to travel back far enough for the bolt catch to lock the bolt back.  In most cases the stock should drop in and run but sometimes you need to do some adjustments.  It is common to think that if an AR part drop in, it will run but it is not that simple.  When it comes to barrels every manufacturer have their own gas port size so it is hard to standardize.  There are of course other variables too from ammunition type to shooting suppressed or unsuppressed.

I just had a similar experience but on 7.62x39

There are 2 things you can try before drilling out your gas port:

1) knock out the pin of the buffer and remove the tungsten weight, which is about 1.4oz

2) if that doesn't work you can try to cut the coils one at a time of the help spring in th buffer, I had to cut 5 coils to get my system to work.

Or you can start drill the gas port, measure your existing and drill in small increments

Here's some pics to show you what I'm talking about, below is the sbr in question, though it's a little unique because it's a piston:
To make it work reliability I tried in order:

Take out tungsten weight
Drill out gas port from .07 to .091 to .125
Add a krink device.

All of this still did not work on 7.5" barrel,
Then I switched had to:
switch to 8" barrel and drilled out the port to .125
Cut the buffer helper spring to the minimum length

Then it was holding then bolt back properly. After I added the tungsten weight back in and it worked properly

Hope this helps










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We don't recommend modifying the spring in any way, especially the inner spring.  

Adjusting the gas port diameter is definitely a key area to look at.  If still under gassed, a muzzle device that provides some back pressure will help as well.  We are able to get the VERT BUFFER system to run on Federal 220gr 300BLK subsonic ammunition unsuppressed in full auto without buffer modification with pistol length gas system and the right gas port size.  That's about one of the worse case scenarios. We have not tested 7.62x39 yet.

OP, how did you get the buffer apart to clip the spring?

Did you end up fixing the problem? What was it?

Nobody has mentioned gas block alignment yet... I would check that before making permanent adjustments to the barrel.

Came to post this ^^^^.

Gas block misalignment is one of the biggest failures I've been seeing with the new generation of AR15 DIY projects that won't function correctly, along with aluminum gas blocks on CLGS or MLGS, followed by hammer springs being installed backwards, gas tubes binding or backwards, backwards crush washers, barrels not indexed correctly after applying torque to the barrel during installation of the barrel nut, improperly installed extension index pins from manufacturers, tolerance stacking on BCGs and carrier raceways in the upper, tool-marked bolt bores in the carriers...I could go on and on.

First thing I look at is how the gas block was installed.

The more we move into the future, the less and less AR15 DIY'ers seem to know about the original design with the handguard cap spacing for the FSB, which drove gas block manufacturing dimensions.