Over or Under Gassed?

[NightFisher]

I have been having a problem on my Grendel build. About half the time when the last round is fired the bolt only partially locks back. Instead of the bolt catch locking on the bolt face it catches on the front of the bolt carrier. Some info on my build: Spikes Tactical Lower, PSA upper, 18" JP barrel with a rifle length gas system (RLGS), adjustable gas block, JP Silent Capture spring with standard spring set.

Some of the things I have done:

• Gas block was initially set almost closed and opened a half turn at a time looking for where the bolt reliably locked back. I have it pretty well wide open now. I made measurements on where the gas port is and verified alignment.
  
• I tried the upper on another lower with the same results.
  
• I have another adjustable gas block from Odin Works on order.
  
• I just got a spring set from JP for the SCS but I have not tried the other springs yet.
  

Yesterday I was working on my son's Grendel which was running perfectly with the Hornaday Black 123's and tried my reloads. This rifle has a 20" Barrel, a non-adjustable Gas Block, and a standard rifle length buffer system. His rifle started locking back just like mine had been doing so it has something to do with my loads. I am running a Hornaday 123SST over 28.2 Grains of IMR8208XBR in Hornaday cases and a CCI #41 Primer. My MV is averaging 2518 which is close to the 2500 I see out of the Hornaday Black factory ammo. I still think I can get this load functioning if I can just tune my Gas System/Buffer system right. Should I start with the assumption that I am Over or Under Gassed?

[grayfox]

There are others on here who have more experience, but I can weigh in with what I've seen on here/ experienced myself.
Same results out of 2 independent rifles does point to the cartridge, yes, it's the common denominator.
Btw how many rounds downrange do you have for this JP build...

IMO the XBR at 28.2 might be maxed out for you, but it's an iffy powder for me for the 120 class anyways b/c it will pressure spike quickly behind those pills when you get over or close to 28.5 (depending on the uniqueness of the barrel). Doesn't give you lots of maneuvering room...
If you have CFE223 or AR comp you could try some loads with them, the 123 sst can be real picky in some rifles. Several have reported lately that it's (sst) not working real great for them. Some folks, it does well, others not so much.

Or try another load... what do the factory loads do for you? 123 Black, Federal 90 TNT's, ---
Or do you have some Sierra MK's (120 or 123 iirc)...

Change out one component at a time to see what happens.
Another thought is the 123 or 100 Eldm's... XBR does fine for me in the 100's.

[StoneHendge]

With that much XBR and wide open gas, I would think your overgassed. If you were undergassed, you likely would have gotten an FTF somewhere along the way. Maybe try the lighter spring which will enable you to turn the gas down and reduce the return force.

[Klem]

It sounds like your hand-loads are not giving enough gas to cycle those guns, so undergassed.

Looking at Quickload I can see that while 8208 is giving you high peak pressure by the time it gets 10" down the barrel to where the port is it has dropped more than other typical powders used with the 123SST. I would keep 8208 for bullets under 120gn and use something like H4895. It will also give you a lower peak pressure for the same velocity, and your gun will thank you for it.

[LRRPF52]

You shouldn't have any gas dialed down on 18" RLGS Grendel from the start. Leave the port fully open as the dwell time is very minimal due to Grendel bore diameter and pressure relief close to the muzzle.

What bolt carrier do you have?

28.2gr of XBR under a 123gr is plenty to drive all Grendel gas system lengths/barrel combos in my experience, so something is off here, especially since you tried it in a 20" and got the same results.

Did you try different mags?

I know some of my old CProducts mags won't lock back correctly, whereas others like Elander lock back fine.

[Sticks]

Or your gas system could just be getting carbon clogged.

I have been having the same problem with my 24" Lilja, AA2230 reloads, and ADJ gas block. After 100 rounds, I start to get FTFeed and does not lock back on the bolt. Adjust the gas a few clicks, and 100 rounds later, same thing.

[A5BLASTER]

Op, I have the same barrel bolt and buffer setup as you, plus have a adjustable gas blocks as well.

I can't speak to your handloads but the first thing I would look at is the gas block.

JP states the gas blocks should be hard against the shoulder of the gas blocks area on the barrel if you have any gap there you are not lined up.

With factory 123 grain rounds adjustable gas block, JP and and PF bcg my gas is set about 3/4 open.

It's a quick and easy, so I would check that first.

[NightFisher]

I installed that gas block hard up against the shoulder like JP recommended. When I started having problems that was my first thought. I pulled the GB and measured the port in it versus the port in the barrel and found that hard up against the shoulder was the correct placement. I am currently waiting on an Odinworks adjustable GB that I want to try before I start changing springs or loads.

[A5BLASTER]

Wouldn't hurt to check the line up of the gas block tunnel to gas tube, inside the gas block.

I had a gas block that when the yuve was installed it wasn't drilled properly and wasn't lined up even though the gas block was properly lined when installed on the barrel.

That's why I only use jp gas blocks now and have the gas tube installed at jp' s shop.

Might check that as well it's anouther quick and easy.

[NightFisher]

Mine was assembled by JP as well. Too late to check the tube alignment now, pulled the gas tube out in preparation of the arrival of the OdinWorks GB. As I remember the carbon on the tube looked normal for a correctly aligned port.

[centerfire]

Have you checked your mag springs? That's a healthy load; the carrier may be moving faster than the magazine follower can lift the BHO to catch the bolt face. Did you perchance use the same magazine in both rifles to test the ammo?

[NightFisher]

Good thought, I only had two Elander mags and I do not have them marked so I can tell them apart. I recently acquired another mag (not sure of the mfg) that I can try as well. Got some stuff to try when I get the new Gas Block on and have the rifle running agian.

[BluntForceTrauma]

What about tweaking your load until it works in your SON'S gun, and then test that load in YOUR gun?

That might be simplest way to rule out the load as the problem before you spend a lot of money on other stuff.

Or try different magazines in your SON'S gun to rule that out, as well.

[NightFisher]

I believe it was over gassed.... Started today with the Gas Block open about a half turn, it would lock back most of the time so I opened it another half turn and got consistent lock back after that.

[BluntForceTrauma]

Strange, I thought when you started you had it closed quite a bit and it didn't work. If I got you correct, you've opened it even more and now it does work? Seems to me that's opposite of over-gassed — based on your first report? I'm missing something. . . .

[NightFisher]

When I first adjusted the GB I started with it almost closed but I had to open it up several turns to get it functioning with factory loads. It was so far open that I called JP to make sure I read their instructions right, GB hard up against the shoulder. New OdinWorks GB started operating the rifle at 1/2 turn open.

[Sticks]

FWIW - Surpurlitave Arms AGB is 4.5 turns to full open. Not sure of JPs specs but they could be close.

[A5BLASTER]

When I first adjusted the GB I started with it almost closed but I had to open it up several turns to get it functioning with factory loads. It was so far open that I called JP to make sure I read their instructions right, GB hard up against the shoulder. New OdinWorks GB started operating the rifle at 1/2 turn open.

Sounds like something wasn't lined up correctly with the jp gas block, mine has never needed to be opened that far to get correct function and lock back.

But good to see your making progress with the other gb.