New Stick, some issues

[bruceleroy]

Long story short...
Rifle: PSA 18" upper with already owned lower. 100 rounds through barrel.

Problem 1hooting American Gunner in the 2" group range. Not what I want.

Problem 2: my trigger, a Rock River National Match 2 stage has such a light and short reset that I am double firing

Problem 3: bolt not locking back, could be related to the trigger as all mags will lock the bolt when manipulated by hand.

First fix was to bed the barrel. After establishing a new zero I shot a 3 shot 0.4" group. Kept shooting and added 12 more rounds to the group expanding it to 1.5" for all 15 shots. I was not rapid firing as I must single load due to double firing but I didnt let the barrel cool much between shots. Maybe 8 to 10 seconds to load new single round mag. I imagine this is about all I will get from this ammo rifle combo.

Next fix will be a new trigger. I am more than happy to accept recommendations for decent triggers that wont have this same issue.

Lastly, a question on the inability to lock the bolt back. Even with the adjustable gas block opened almost all the way, and several spots in between, I cant get a consistant lock back of the bolt. What the F! I meed recommendations on this please.

Update: I threw a new trugger in, standard GI, while waiting on a Larue that I ordered. No double fires. Adjusted gas block to half of the available turns. It now locks back.

[kpswihart]

Problem 3 could be a couple of things. My first thought would be issues with gas, I'd start there. I'd also take a look at the buffer and spring.

Before I spent money on mitigating Problem 2, I'd make sure I was following through on individual shots. I'm no high-speed operator, but I've had enough training and gotten enough experience to know that follow through is important. Make sure you're not getting off the trigger, following the shot, too quickly. After the shot, get off the trigger slowly and consciously feel the reset. Slowing down a little may help with both 1&2.

[Bigs28]

Check to make sure the gas block is aligned over the gas port. If it is then the buffer could be an issue or the gas port size could be the issue.

Trigger..... what kp said or sell it and get a larue

[VASCAR2]

The LaRue MBT-2S trigger comes with two hammer springs (heavy, light) which might help. I’d double the RRA trigger to make sure the springs are installed in the correct orientation. If you take the gas block off measure the gas report. I read a report where one person’s gas port was to small. The Faxon 18 mid length Gas system 6.5 Grendel uses a .076” size gas port and the PSA gas port should be close to that size. Some people use drill bits to try and measure the gas port but you can probably get pretty close with a set of calipers.

The PSA adjustable gas block have two screws, one for adjusting the gas and the second for keeping the adjusting screw from coming loose. Verify the adjusting screw is moving, screws can strip out or some people were turning the wrong screw. There have been at least two different adjustable gas blocks used on PSA 6.5 Grendel barrels. One AGB has the adjustment screw on the muzzle end of the gas block and the other AGB has the adjustment screw on the side.


It is possible your failure to lock back could be attributed to a small gas port, improper setting on AGB or a to heavy buffer and spring. With an adjustable gas block I use a carbine buffer and carbine spring in my 6.5 Grendel AR-15’s. I’d weigh my buffer to determine which buffer I’m using as not all buffers are marked.

[FRB6.5]

I'd ensure that there is no grease or oil on the trigger and hammer sear engagement surfaces. I was getting doubles from the bench with a LaRue when I greased them by accident.

[bruceleroy]

Follow on questions and statements:

Is there an ideal buffer weight for the grendel or is it just trial and error? The current setup is all carbine hardware.

The trigger is very clean, it is just the shortest reset I have ever seen...ever.

As far as trigger discipline, I am very trained in trigger manipulation. I have never had a trigger double on me like this one will. It will even do it when firing a 5.56 with a muzzle brake. It is a stupid short reset.

The gas port is definately not small. I will get it measured and post details. The gas block is the newer model with one adjustment screw straight out the front. I closed it and started to open until I got to about 5 turns open and it wouldnt lock open.

[FRB6.5]

There isn't a specific "Grendel" buffer weight. It is just one tuning factor that has to work in concert with; the particular cartridge load, barrel length, gas system length, gas port size, spring weight, BCG weight etc. Changing one of those can require adjustment somewhere else in the system. An 18" mid-length gas system would likely use an H1, rifle length a standard carbine buffer. Since you have adjustable gas you shouldn't need to change the buffer.

I'm not familiar with your specific trigger but it sounds like it may be trying to clone the function of the Geissele S3G 3 gun triggers. They are meant to provide the most rapid fire possible for close course of fire use. They aren't suited for duty use, bench use or accuracy work for the reasons you are experiencing. Although "2 stage National Match" as the name of it would imply different function and purpose.

Happen to notice where the brass ejects?

[bruceleroy]

Brass is at 3:30 to 4 oclock every time.
I will be weighing my buffer and measuring gas port today.

[Bigs28]

I had a lower that had a siezed bolt catch that i had to take out and clean everything real good. Take your lower off and see if your bolt catch comes up when you insert an empty magazine.

[bruceleroy]

Just checked the bolt catch. It moves freely when mag is inserted.

Buffer weighs approx 2 1/2 ounces and gas port is approx 0.09 on the money. Seems that this is fairly standard looking at other makers. I should be able to overcome these issues with the adj gas block.

[FRB6.5]

3:30-4:00 ejection would indicate your gas system is fine. The buffer weight is light though, 3 oz is standard carbine. But the ejection pattern seems to indicate that it's workable with that combo. I'd think if it was letting things unlock too early you'd see bulging on the brass and typical 1:00-2:00 ejection.

I assume you have a 5.56 upper you've used with that lower before. Perhaps try swapping the bolt carrier between the uppers and see if the problem goes away/moves. Possible tolerance stacking issue and that PSA carrier is dragging.

[bruceleroy]

I did some more investigating on the adjustable gas block. I was under the impression it would only come out 5 turns from bottom but it will do 8 to 9 full turns out before I would not feel comfortable with how much thread is still engaged.
I will play some more with that and swap the bolt carrier with my 5.56 upper.
Trigger is also on the way.

[A5BLASTER]

I did some more investigating on the adjustable gas block. I was under the impression it would only come out 5 turns from bottom but it will do 8 to 9 full turns out before I would not feel comfortable with how much thread is still engaged.
I will play some more with that and swap the bolt carrier with my 5.56 upper.
Trigger is also on the way.

If your adjustable gas block is opened that far and your gas port isn't undersized.

Your gas block is not properly positioned over the port.

Anytime you have to open a adjustable gas block past half open, your gas port is under sized are your gas block is not centered over the gas port.

[FLshooter]

I own close to a dozen Larue MBT 2S triggers and will probably own more. Best bang for your buck as far as triggers go.

[CWOZ]

FWIW:
I have a PSA 18' stainless upper I bought as a blem. Looked just fine when I got it, couldn't figure out why it was a blem. When I took it out it would not cycle Wolf, nor any other factory ammo I tried with the side screw on the agb way out. Did some looking online and the 18" stainless barrels supposedly had tight gas ports, so I opened it from .083 to .098. works like a champ with everything I have tried with ajb about 3 or 4 turns out. (Been a while since I did the adjustment so not sure if it was 3 or 4 turns).
Not sure if opening the port was the correct action, but it cycles fine with the standard carbine buffer and spring.