Handloads Short Stroking

[lesslaw]

I built an AR lower for a friend using a standard carbine buffer and buffer spring. It's the same set up I have used on lowers that I own running 5.56 and 6.8 SPC (both 16" barrels). My friend added an 18" Grendel upper from PSA. (PSA 18" Rifle-Length 6.5 Grendel 1/8 with adjustable gas block) The rifle handled factory ammo from Alexander Arms (120 grain TSX) but it is short cycling on handholds using the following charges:

Barnes TSX 120 grain using H335 powder and charges of 27.5 - 28.0 grains
Barnes TSX 120 grain using 8208 XBR powder and charges of 27.5 - 28.0 grains

All rounds eject but do not load next round or lock back bolt on empty mag. All loads were near or at max charge and I believe they should have cycled the bolt if all other pieces (buffer, gas tube, gas rings, bolt lube, etc. etc.) are working properly. When he fired the Alexander Arms factory rounds, he said he felt a noticeable (hotter) difference from the handholds.

1. Should I replace the buffer and spring with a different set up than carbine?
2. I have read that some PSA uppers had some issues with the gas system. Should we contact PSA?
3. I have never run an AR with an adjustable gas block. Should I have him make sure this is wide open?

Suggestions and advice are greatly appreciated. Thank you in advance.

[A5BLASTER]

Try adjusting the gas block. Adjustable gas blocks are used to tame over gassed rifles.

If he set the gas too the ammo he used, it's a good chance he will have to reset it to the handloads.

One of the draw backs of adjustable gas some times you have to adjust it for one load and if you change loads are try diffrent factory ammo you may have to readjust.

[Popeye212]

I had to do the same thing with a Creedmoor. Used Hornady Black to adjust block but had to readjust with my handloads. It was only one click but that was with that rifle. YMMV

[A5BLASTER]

I had to do the same thing with a Creedmoor. Used Hornady Black to adjust block but had to readjust with my handloads. It was only one click but that was with that rifle. YMMV

I found if I set my gas with wolf fmj. Then the gap between running and not running gets a lil wider with handloads.

But I still have to adjust for most of them.

[bbqncigars]

It definitely sounds like the gun is under-gassed. Open it up with one round in the mag until it locks open, and give it a little more. This is what I've done with all of my adjustable gas guns.

[Kswhitetails]

OP- Gas should be fully open any time you're changing loads. This sets your baseline and ensures the system functions as designed without being hampered by less than full gas. The only reason to choke your gas is when you're over-gassing the system to the point where it fails to function in some way.

Disclaimer: I do not run adjustable gas blocks in my Grendels. I have at best a solid rudimentary understanding of the AR DI system.

There are three things, as far as I can tell, that need to be considered when setting up the gas system on a gas operated rifle/weapon.
1)Burn rate of the propellant. 2)volume of the gas generated by said propellant 3)pressure generated by the detonation cycle, within the system - as built.

This is understanding and assuming the other "standard" variables. System length, port size, buffer weight/spring weight, plug mass (bullet weight) are all within the "normal" window of operation.

A lot of guys insist (folks I mean no disrespect or offense here, bear with me) that an adjustable gas block is necessary when building Grendel. I'm not sure why. BA was picky when he started settling with final versions of the chamber, propellant selection, and projectile usage for Grendel because he wanted it to work well with the gas systems that had already been designed and proven on the 5.56 AR. When we start tinkering with the variables, we start finding points where we are now not able to use the system as designed. We've moved outside the proven envelope of which the system is known to perform reliably.

So, simply put, this isn't a problem looking for a solution until we start changing the standards with which we operate. Choking the gas, changing the buffer weights, the BCG mass, bullet selection, etc... makes the entire equation fail. Now, we must do the work to determine the limits of the new window in which we operate. We must - out of necessity, put in the work. I mean to say, do - the experimentation, data collection, and variable identification and set the new definitions. All of which are now required to determine our new operating "window" for the gas system.

If I had my 'druthers, I'd start back at the beginning. A standard gas block, buffer, BCG, and spring. If I had to bet on it, the Grendel would show more promise than folks allow it to. Have some faith. BA did EXTENSIVE testing to ensure that Grendel would perform well under the standard configurations already proven with millions of rounds down range on other chambers and calibers.

Otherwise, just learn the DI system. It's just a physics lesson on pressure vessels, pistons, vents, and the related dynamics. Understand the physics, then adjust the system as required to ensure enough pressure, volume, and dwell time are met. It'll work then, I guarantee it.