Help needed with FTF and Double Feed issues using 129 gra ABLR

[grendelnubi]

I have had some double feed, failure to feed and short stoke issues with two of my Grendel's, the 12.7 and 18".

At first I thought it was a mag issue since I just purchased a new 10 rd ASC mag. The new 10 ASC is hard to seat up into the mag well, really have to hit it harder than normal to seat the mag so I was convinced it was a mag issue. Second time to the range, I tested other mags and the issues continued. I have never had any issues before it just started and is only happening if I am shooting my new 129 gr ABLR load. Other projectiles work just fine. This is leading me to believe it is a gas issue. I have opened my gas block up one turn on both Grendel's and will see if this fixes the issue.

I am not an expert on understanding the gas system and how the projectile weight impacts cycle rate, dwell time and lug timing. My 18" is a MLGS and my 12.7 is a CLGS. It appears to me it is under gassed when using the heavier weight projectiles. Hopefully I am on the right path with opening the gas block a little.

Any suggestions in understanding and addressing the issue would be helpful.

[A5BLASTER]

What buffer spring and buffer for each barrel length?

[grayfox]

what's the ejection pattern, what "o'clock" does the brass eject at.
And does this happen also for other loads/rounds?
Off the bat I'd suspect a cleaning issue, some carbon build up in the bcg, bolt, on inner surfaces of the upper, or in the chamber... or some oil/crud build up that's slowing stuff down. If it works with others or if it just started happening, do a clean/inspect routine then try the rounds again.

[BCHunter]

If your mag isn't seating properly it will cause FTF. Push hard on the mag release button, and back off the mag catch one turn. This should help the mag seating issue.

What is your load that's not working?

[grendelnubi]

Thanks for all of the comments;

I am using the standard AR buffer and springs in both my Grendels. Please note the issue is occurring on two different AR's in the same exact way shooting the same projectile and load. I have never experienced any issues with either of these AR's except when shooting the 129 ABLR. My 129 grain ABLR load which is 26.9 grains of AR Comp and my 18" (MLGS) is running about 2353 FPS. I estimate by 12.7 (CLGS) is running 2150, the load is pretty fast IMHO.

After the first range time issue I did clean it very thoroughly and issue reoccurred the next trip to the range. Again, only with the 129 ABLR.

I do not know my brass ejection direction, I was using a Caldwell brass catcher but next range trip I will retest again and see what direction the brass is being ejected out at. I have already opened up both GB's one turn so I am hopeful the issue is fixed.

When I adjusted by GB after building these Grendels, about 2 years ago, I adjusted my GB till the bolt locked back then made another 1/2 turn. I should have opened it up at least one turn. I am convinced this heavier 129 grain projectile must have an influence on the gas system some how, I just don't know how it affects it. In my thoughts, I would think the heavier projectiles should result in more gas pressure at the GB but I really don't know for sure.

[FRB6.5]

2 years of use could cause enough carbon build up to reduce gas flow where a lower pressure load won't cycle it, especially if it was adjusted to a minimum setting with a higher pressure round.