6.5 Grendel Cycling Issues

[Stugots]

I have begun working up loads for hogs with my AR15 6.5 Grendel build. The problem I have been having is after I fire a round, the case fails to eject. I have an adj. gas block which is basically wide open at this point. Barrel is a 22" with a rifle length gas system. I began working up reloads using 26grs of Alliant Power Pro Varmint using 123gr Hornady SST's. My heaviest charge is 28grs. For each 5 shot string, I inspected each case for signs of hi pressure. (Saw no indications with excessive pressure).

The issue I have is not 1 single case was ejected after the shot. In many cases, I had to really pull hard on the charge handle in order open the chamber and remove the spent brass. I finally had a friend watch the BCG while I took a shot down range to see if the BCG was moving at all. He said after watching me take 8 or so shots, the BCG did not move at all or moved back very little. (My guess is it may be related to head space issues...)

I have built 15+ AR's mostly .223 with 1 rifle chambered for for .204 Ruger, and 2 AR10's (1 chambered in .308 and the other, 6.5 Creedmoor) and everyone of them ran flawlessly.

Any ideas on a course of action to figure this out?

 

[37L1]

If the BCG isn't moving, it sounds like you are not getting enough blow back to operate your system.
Check gas block alignment? Do you get air to pass down the tube, through the gas block and into the barrel?
Measure gas port in barrel?

 

[hlee]

What buffer and spring? Mine run with a standard carbine buffer amd spring.

Perhaps dumb, but make sure you are adjusting the block correctly (ie, that it is full open and not full closed).

Lube everything well. Doesn’t need to be sloppy, but should be wet.

Insure gas port is aligned with gas block.

 

[Stugots]

Perhaps dumb, but make sure you are adjusting the block correctly (ie, that it is full open and not full closed).

Insure gas port is aligned with gas block.

Gas block was adjusted to wide open.

I just pulled the handguard and gas block and noticed the port on the gas block did not perfectly align up with the gas port on the barrel. I believe that should solve my problem and will let you know after I have a chance to go to the range and shoot it later this week. Thx for the reply!

 

[drewthebrave]

I just pulled the handguard and gas block and noticed the port on the gas block did not perfectly align up with the gas port on the barrel. I believe that should solve my problem and will let you know after I have a chance to go to the range and shoot it later this week. Thx for the reply!

That's the most common cause for this type of malfunction. Proper alignment should give you consistent ejection.

 

[r.tenorio671]

That's the most common cause for this type of malfunction. Proper alignment should give you consistent ejection.

...FWIW, a simple way to "better align" a gas block in a new build is to take the measurements from the back of the barrels gas port to the shoulder step between the breech and back of gas block. Do same for the gas block, rear of gas port to rear edge of gas block. Compare the 2. Just based on my personal experiences, the gas port in the majority of gas blocks I've used is typically larger in diameter than the barrels gas port, so some "wiggle room" is available. Another quick alignment aid when using set screw type gas blocks is rotate the gas block on barrel so the rear set screw hole is aligned & centered over the barrels gas port, using a straight edge mark a line thru the center of the gas block screw holes to the barrel to use as a reference mark when gas block is rotated back to normal position. Also, while the gas block is rotated in that reverse position, check to see if there is a gap between the back of the gas block and the shoulder step of barrel/gas journal junction. If there is a gap, feeler gauges can be used to measure it, but actually anything that can slip between the shoulder and gas block will do (business cards, etc.)

 

[MRattpack]

What buffer and spring? Mine run with a standard carbine buffer amd spring.

Perhaps dumb, but make sure you are adjusting the block correctly (ie, that it is full open and not full closed).

Lube everything well. Doesn’t need to be sloppy, but should be wet.

Insure gas port is aligned with gas block.

 

[Stugots]

I'm using a JP Silent Captured Spring. I'm using this exact system on 4 other ar15 platforms Identical weight system and all. The other 4 AR platforms run flawlessly.
I did get a chance to sneak away from work this afternoon and chrono some loads for my hog hunt down in Tx this January. On a positive note, all of my loads grouped 1 MOA or less. The BCG is still not coming out of battery enough to eject the spent shell casing. I asked a fellow shooter to video the action while I took a shot down range and just watched the vid. As I stated, the action is opening. I went through the vid frame by frame and estimate the action opens approx. 1.35-1.4". I live about 40 miles south of JP Enterprises and plan on making a trip up to their offices (Silent Captured Spring in tow) and asking them if they can break it down reducing the weight of it and seeing if that will make a difference...

 

[Stugots]

r.t671, Tomorrow I will pull the handguard again and take measurements on the port of the gas block relative to the barrel port. Any other suggestions? TIA Guys, Stu!

 

[r.tenorio671]

r.t671, Tomorrow I will pull the handguard again and take measurements on the port of the gas block relative to the barrel port. Any other suggestions? TIA Guys, Stu!

...the one thing you identified that has me kinda puzzled is that "The issue I have is not 1 single case was ejected after the shot. In many cases, I had to really pull hard on the charge handle in order open the chamber and remove the spent brass." Did the cartridge chamber normally with no issues? Typically the case should have shrunk back down AFTER having been fired. Because your observation that the BCG doesn't move or moved very little when fired would normally make me think you have a gas issue, basically not enough gas getting to the BCG or the gas key is leaking, but....having a hard time extracting the casing makes me think you may have a rough chamber that is causing the casing to "grip" more when it expands, which could delay the movement enough that it won't cycle properly because the gas impulse passed already. You may want to check the chamber. If the chamber is really rough you may want to contact the manufacturer/seller.

...also, if you have a standard carbine buffer spring laying around try swapping that in to see if it functions with it. If it does then you have info to pass to JP that they can use to advise you on what if any modifications would be needed to your SCS.

 

[Stugots]

r.t671 Yes, the cartridge did chamber and I was able to eject it by pulling the charge handle back without any issues. As I had mentioned in my last post, the video I shot showed that the BCG moved back approx. 1.35-1.4" after each shot. I have a bore scope and will take a look at the chamber to see it is rough or not. I will also inspect the gas port from inside the barrel and verify I have the adj. gas block properly a aligned.
I looked on line online at a set of go/no go gauges and WOW, they are not giving a set of those away!

 

[mtrmn]

Will factory ammo make it work?

 

[Stugots]

I believe I found the root problem. I measured the distance between the edge of where the hole was bored through the barrel allowing the gas to vent into the gas block and to port in the gas block. If I seat the gas block against the the shoulder of where the gas block seats against the barrel the gas block port extends beyond the barrel port by approx. 1mm. (Please see the attached pix.) I tightened down the gas block 1mm prior to where it engages the shoulder of the barrel and it centers over the port on the barrel perfectly. So who is at fault? The gas block manufacturer for incorrectly drilling the port in the block, or the barrel manufactured for incorrectly drilling the gas port in the barrel 1mm further down the barrel (closer to the muzzle of the barrel?!?

 

[Stugots]

Will factory ammo make it work?

Unfortunately I haven't been able source any factory ammo

 

[hlee]

If you are using a gas block designed for use with an A2 style handguard, you may be at fault for not understanding how to set up your chosen equipment…

 

[Stugots]

If you are using a gas block designed for use with an A2 style handguard, you may be at fault for not understanding how to set up your chosen equipment…

I'm using an adjustable gas block on this build which is the same one I have used on 4 other uppers. Over the years I have built 9 AR uppers (as well as a few AR 10's along the way) that I presently own (with a few sold) and have built/assisted a few of my friends in another 12 or so builds. They have all with the exception of this build ran flawlessly. I believe I have acquired a rudimentary understanding of how to correctly assemble an AR 15 upper.
Thanks for your input...

 

[mtrmn]

The reason you dont mount the gas block flush with the barrel shoulder

This comes up very often. Guys take there new gas block and butt it up to the shoulder of the barrel. This is incorrect unless stated differently in your gas block instructions. The reason is the proper gas block location allows for an A2 handguard endcap to be placed between the gas block and ba...

forum.308ar.com

 

[r.tenorio671]

I believe I found the root problem. I measured the distance between the edge of where the hole was bored through the barrel allowing the gas to vent into the gas block and to port in the gas block. If I seat the gas block against the the shoulder of where the gas block seats against the barrel the gas block port extends beyond the barrel port by approx. 1mm. (Please see the attached pix.) I tightened down the gas block 1mm prior to where it engages the shoulder of the barrel and it centers over the port on the barrel perfectly. So who is at fault? The gas block manufacturer for incorrectly drilling the port in the block, or the barrel manufactured for incorrectly drilling the gas port in the barrel 1mm further down the barrel (closer to the muzzle of the barrel?!?

...really not the an error on the manufacturers part, remember, the "standard" M16/M4/AR15 uses a plate between the A-frame front sight/gas block and shoulder of barrel/gas journal junction for the typical non-free floated handguard. With the advent of free-floated handguards and lo-pro gas blocks the GI issue A-frame front sight/gas block is rarely used anymore having been replaced by front sights that attach to the 1913 pic rail of handguard. Some gas block manufacturers make the port in their gas block "oversized" in diameter so that if the user butts the gas block up against the shoulder, it will still enclose the whole barrel gas port, some, not all do. It's on the user to check these measurements before assembling.

...Edited: FWIW, I know of folks that had over-gassed systems to begin with that assembled their gas blocks against the shoulder whereby it partially covered the barrel gas port, effectively "restricting" gas flow enough that the platform would function ok....until the hot gasses eventually eroded that overlap and they began experiencing issues requiring changing the buffer/buffer spring or replace with AGB's to mitigate the now experienced over-gassing issues. YMMV

 

[Stugots]

Just got back from the range and the rifle functioned perfectly. Not a single failure to eject a single brass case. What I really find amazing is the fact that ALL of the loads I worked up for this build into an inch or less (center to center). I had a couple group into 1/2" C to C.