Problems with 6.5 Grendel (Accuracy & cycling issues) Need Help

[Ryder Bohannon]

So I just built a 6.5 Grendel, bought the complete upper from Grendel Hunter (18" Mid Length) and I've taken it out to the range 4 times now. Every single time out it's had problems cycling and feeding the ammunition and the accuracy has been really bad. Sometimes the bolt doesn't want to lock back on the last shot, it's getting stuck trying to pick up the next round and sometimes not picking it up at all. Groups are averaging 2.0 MOA at 100 yards.

My brass is getting absolutely ate up by this gun, it's causing major dents around the neck of each round making a "D" shape in the case necks. When I measured the brass with my headspace comparator it grew from 1.210" (new) to 1.222-1.226" (fired) on average and the loads that showed pressure signs grew to 1.228 - 1.230"!!!!!! I was concerned that it could be excessive headspace so I checked it with a Go/No Go Gauge and they both checked out fine.

I was also running an H2 buffer and thought it might be short stroking so I put a carbine buffer in and it still did not want to cycle. The upper came with an adjustable gas block so I might need to try tuning that next. I'm using an ASC mag right now and I just ordered an E-Lander mag to try and see if the cycling is a magazine issue.

As far as accuracy I thought the problem could be a scope issue so I put another scope on the gun and it shot just as bad so it's not a scope issue. My next move is to buy some factory ammunition and give that a try. 95 rounds down range so far and not a single group below 1 MOA. I Did ladder testing with AR-Comp and IMR 8208 from 26.3gr up to 28.5grs. It seems like I can't go above 28grs with AR-Comp without getting pressure signs. I was able to go a little higher with IMR 8208. The gun didn't shoot either of the powders, bullets or primers well. Going to back down the loads again to 26-27.5grs and re test after testing factory ammo first.

Results were as follows
Load 1
Brass - Lapua / Powder - IMR 8208 /Primer - CCI 450 /Bullet - Hornady ELDM 123gr /COAL - 2.250"

Powder grains - 5 shot group (MOA)
26.9 - 3.2
27.2 - 1.9
27.5 - 1.1
27.8 - 2.2
28.0 - 1.7
28.1 - 2.3
28.2 - 1.7
28.4 - 1.5
28.5 - 2.1
Average = 1.96 MOA

Load 2
Brass - Lapua /Powder - AR-Comp /Primer - CCI BR4 /Bullet - Hornady ELDM 123gr /COAL - 2.250"

Powder grains - 5 shot group (MOA)
28.0 - 3.2
28.1 - 1.4
28.2 - 1.6
28.4 - 1.2
Average - 1.85 MOA

Load 3
Brass - Lapua /Powder - AR-Comp /Primer - CCI BR4 /Bullet - Sierra Match King 123gr /COAL - 2.250"

Powder grains - 5 shot group (MOA)
27.5 - 2.3
27.8 - 2.2
28.0 - 2.3
28.1 - 1.6
Average - 2.1 MOA

Dented Case Necks

Ejector Marks on case head

Bulging case head (The one on the right is the best example)

Ring of brass I got after trying to resize those cases

Best I could get at 100yds (Red dots = 1/2")

 

[Carlos Danger]

As far as cycling, maybe check seating depth. I ran handloads seated .020 off the lands and then realized I wasn't paying attention to mag length. (Idiot mistake) Seated too long. IIRC my jump is damn near .208" at published COAL. I'm not a gas gun expert.

I have an ER Shaw bull barrel and have had respectable results.

I don't know if my brass is dinged at the volume level you posted, but it does happen. No resizing issues (Hotnady dies) Can't speak to the length issue but don't recall mine needing that much trimming. Mine is an old Model 1 Sales upper / barrel setup. Maybe yours is chambered "deep"?

 

[Middle Man]

Over gassed? Loads likely too hot?

Yes, test with factory Hornady 123 loads.

Certainly see signs of excessive pressure in your third photo

Case mouths are being dented on the case deflector upon ejection is not uncommon

 

[Carlos Danger]

I ran a ladder of 123's up to 27.6 of 8208XBR with no pressure signs.

 

[Silverjay]

That brass is showing pressure signs, especially for a Grendel. Book max is 52,000 for the Grendel. I run 8208 under 123 Nosler CC’s in two different Grendels and brass doesn’t look like that. These are right at 52k pressure according to quickload. These are loaded at 2.250 COAL and cycle well in both of mine.

 

[Ryder Bohannon]

That brass is showing pressure signs, especially for a Grendel. Book max is 52,000 for the Grendel. I run 8208 under 123 Nosler CC’s in two different Grendels and brass doesn’t look like that. These are right at 52k pressure according to quickload. These are loaded at 2.250 COAL and cycle well in both of mine.

I agree just not sure why it’s showing pressure signs, my COAL is 2.250” as well.Are your loads compressed? I get a crunch when I seat the bullet even at 27.5grs

 

[TRG65]

With the marks on your case heads, and the profile shot looking like you are trying to create a "belted" mag, I would dial the loads back significantly. You could check the headspace also with a no go gauge.

 

[wigwamitus]

I had grendel for 4 years ... cycling issues for mid-length gas system I would say are "standard" ... you can address thru a combination of AGB and/or balancing the BCG/Buffer, spring. I used an SLR gas block to solve the issue in my case.

 

[Campguy308]

As others have said, too much pressure.

Check to ensure your bullets aren't being jammed into the lands. Load a round into the chamber, then eject it unfired. (Or just use hornday tool to measure distance to lands). I had a Grendel once that was so short throated, it wouldn't chamber factory Hornady without massive land jam.

 

[Ryder Bohannon]

I had grendel for years ... cycling issues for mid-length gas system I would say are "standard" ... you can address thru a combination of AGB and/or balancing the BCG/Buffer, spring. I used an SLR gas block to solve the issue in my case.

Might need to try an adjustable gas block since I tried a carbine and H2 buffer.

 

[wigwamitus]

Not sayin' you aren't over pressured as well ... just adding in ... that in general mid-length 6.5G systems are "famous" for cycling issues ... that said, I had surprising accuracy ... and same performance out to 900yds as buddies 308 stoner 16 inch system ... the reason I got out was I went 18 months without being able to find the factory load I wanted (FGM 130gr) ... now that I reload for rifles ... that's no longer an issue ... but I've moved on now and standardized on just two calibers (556 and 762) to simplify the logistics chain, less types of bullets, brass, powder, etc.

 

[Robert4]

So that's .012-.015" case growth at the shoulder from new to fired. That's a lot. I'd suggest checking that chamber with go and no-go gauges.

 

[Ryder Bohannon]

So that's .012-.015" case growth at the shoulder from new to fired. That's a lot. I'd suggest checking that chamber with go and no-go gauges.

That's what I was thinking because usually I only get about 0.003 to 0.004" case growth. I also have a case guage and the rounds with ejector marks on the case head would not fit in the case guage due to the swelling at the case head, I believe that's why when I tried to resize them they would not fit into the reisizing die without cutting that ring of brass. Just bought a go and no go guage though to check the headspace.

 

[JS8588]

I'd send Grendel Hunter an email. See what he says.

 

[Ryder Bohannon]

I'd send Grendel Hunter an email. See what he says.

Figured I would at least run some factory ammo through it, but if I get the same results then I will be contacting them.

 

[kingzero]

That sucks. I don't have anything to add based on what's been said already. I built a 16" mid length Grendel recently with a Criterion barrel by Craddock Precision and it has ran perfectly. I was destroying prairie dogs last weekend with it running suppressed in high winds and lots of dirt and it didn't give me any problems. Hope you get it figured out, the 6.5 Grendel is a fun cartridge.

Edit to say I was running factory Hornady 123 SST's.

 

[bfoosh006]

Might need to try an adjustable gas block since I tried a carbine and H2 buffer.

To me... your pics suggest early unlocking of the BCG, while the case is under considerable pressure.

And your charge weights might be high for your barrel / chamber / throat length ... etc.

I would tune the Adj. GB from fully closed to 100% lock back on a single loaded round. Carefully watching your fired brass cases for any issues.

IMHO, use factory ammo for your first testing w/ the Adj. GB... then try your reloads.

I could be completely wrong... but your issues suggest to me, a large gas port.

If you don't have a Adj. GB yet... buy quality... typical $30 Adj GB's aren't so wonderful.

 

[CaptNemo]

I'm in agreement with bfoosh006 - he and I have plowed the same ground with the PSA PA 10 rifles.
Get a couple of boxes of Hornady Black ammo, that's pretty much the standard set up ammo for a Grendel.
If the gas block is adjustable, close it all the way down ( Zero Gas ) fire a round and look at the brass for pressure signs, etc.
Open the block about 5 clicks to start and then put one round in the magazine and fire the rifle adjusting in 2 click increments as you go until the bolt locks open on the empty magazine.
Shoot the Hornady Black Ammo and get back to us.

 

[Darayavaus]

I have all 'grendel 2' chambers and bolts. without researching it, I know the G1 chamber was different, but I forget the specific difference. always make sure that you are running the correct bolt/chamber combination.

 

[Ryder Bohannon]

I have all 'grendel 2' chambers and bolts. without researching it, I know the G1 chamber was different, but I forget the specific difference. always make sure that you are running the correct bolt/chamber combination.

The Bolt is Type II .136 bolt face depth and the barrel is a type II chamber.

 

[Ryder Bohannon]

So that's .012-.015" case growth at the shoulder from new to fired. That's a lot. I'd suggest checking that chamber with go and no-go gauges.

Just checked the headspace with Go/No Go gauges and they both checked out good to go.

 

[sam4886]

Looks way over gassed to me, too. Doing load work up, barrel break in, and new gas gun tuning at the second time is going to kill you every time. The grendel isn't too bad of barrels so you've got time to get stuff figured out one at a time.

Run 100 or so factory rounds to break everything in and get feeding issues figured out. Then do load testing.

I like PRI mags best. Ive tried c products and e lander too, but the PRI are more consistent and better built. They allow for longer coal too.

I was getting similar bulges with XBR, can't find it locally now so switched to benchmark and like it better.

 

[Ryder Bohannon]

Looks way over gassed to me, too. Doing load work up, barrel break in, and new gas gun tuning at the second time is going to kill you every time. The grendel isn't too bad of barrels so you've got time to get stuff figured out one at a time.

Run 100 or so factory rounds to break everything in and get feeding issues figured out. Then do load testing.

I like PRI mags best. Ive tried c products and e lander too, but the PRI are more consistent and better built. They allow for longer coal too.

I was getting similar bulges with XBR, can't find it locally now so switched to benchmark and like it better.

After headspace checked out fine I'm thinking it might be overgassed as well. When I called grendel hunter and told them about the cycling issues they said that I need to turn the gas up! I asked if it would be better to turn it all the way down and work it open until the bolt locks back on an empty mag and they said no that would be a waste of ammo since they think it's short stroking. Regardless I think i'm going to close it down and slowly open it. You're also probably right that I need to get some rounds through it before doing hand load testing.

 

[sam4886]

I went back and looked at your OP, and I suppose it could be short stroking. It's tough to tell without more detail/testing.

Make sure it will lock back by hand on an empty mag. You can also single load and adjust until it locks back on the empty mag after firing. This is just to eliminate any feeding issues related to the mag.

I don't think anyone makes grendel specific mags, they're all just 6.8 mags, maybe with a different follower, maybe not. All mine have taken some feed lip tweaking to be reliable.

You did make sure youve got the right length buffer tube and spring combo right? Never hurts to check.

Either way, as long as you can get it dialed in with factory ammo first, everything else will be easy.

 

[MNfarmer]

Yeah sounds like short stroking. Maybe caused by overgassing which pulls case out of chamber too soon causing pressure to expand the case and holding the bolt up from completely cycling. I would do what others said get factory ammo and adjust gas block from fully closed until it locks back on empty mag. Double check the bolt for possible damage first from possible over pressure. I have had to mess around with settings the most on grendel based cartridges but once I found the sweet spot they all ran great.

 

[eddie102870]

check your mags too. we had some cheaper mags that wouldnt feed good and bolt wouldnt lock back on last round. bought some lancer mags and all was well in the world again.

 

[MNfarmer]

Yeah there are nice stainless mags available with blue followers specifically for grendel. Forgot the name but a simple google search will find them work perfect in my grendel's and 6mm fat rats.

 

[CrabsandFootball]

First Step - Don't buy Junk gun parts
Second Step - Shoot Factory Ammo to eliminate your reloads being the issue
Third Step - Get rid of AGB (they all eventually fail) and run a bootleg carrier
Fourth Step- Try out different Mags, could be a follower angle or dwell issue
FIF Step - Take whole upper apart, reassemble it correctly. If you aren't capable, pay someone to do it.
Six Step - Work on your reloads. You should never be max pressure when starting out. The fact you are getting pressure signs so early, means you are doing It wrong.
Seventh Step - If you still can't figure it out, pick a new hobby, preferably one that won't kill or maim you.

 

[Snuby642]

Over gassed just like my 6.5g, 20 inch upper.

It gave false overpresure signs, and feed problems.

Unlocking too early.

I used a extra strength / lenght flat wire spring, h2 buffer.
No more mashed up brass.

In this condition 1-1 1/4 moa.

Agb fixing to go on to smooth it out further and redo test with 8208 and 123 sst.

Just went through same with 223 except may remove extra power spring.

 

[Ryder Bohannon]

I'm in agreement with bfoosh006 - he and I have plowed the same ground with the PSA PA 10 rifles.
Get a couple of boxes of Hornady Black ammo, that's pretty much the standard set up ammo for a Grendel.
If the gas block is adjustable, close it all the way down ( Zero Gas ) fire a round and look at the brass for pressure signs, etc.
Open the block about 5 clicks to start and then put one round in the magazine and fire the rifle adjusting in 2 click increments as you go until the bolt locks open on the empty magazine.
Shoot the Hornady Black Ammo and get back to us.

Great update - Backed off the powder on my hand loads, shut the gas block down and slowly opened it up. 1 full turn to start short stroked so I opened it to 1 1/2 turns and it ran flawlessly after that.

As far as the accuracy I believe the loads were too hot for my guns liking. 26.0 to 26.8gr IMR 8208 shot sub MOA with the sweet spot at 26.6 and 26.8. Once I got over 27.0grs groups started to open up and recoil stiffened. Thanks for the replies and guidance, much appreciated!

 

[LRRPF52]

There are a lot of after-market assemblers who source 18" barrels with MLGS and ports that are way too large, as in the .094" range.

I lost track of how many 18" MLGS barrels I've seen like that, with unknown chambers trashing brass, and accuracy problems when run semi-auto.

The funny thing is a lot of those barrels will shoot bug holes with the gas shut off, but when you open up the gas even a little to get them to run in the cold, the groups open up immediately outside of 1.2-1.5 MOA, even with bedded barrels into the upper, top quality glass, mounts, rear bag, solid position, and gas gun fundamentals.

An 18" MLGS 6.5 Grendel doesn't need more than .076" when shooting 100gr or more. I own at least (3) 18" Grendels, all of which are MLGS, and lost track of how many 18" MLGS Grendels I've built or trouble-shot. I also have 16" MLGS Grendels and have built a lot of them too. None of my MLGS Grendels have adjustable gas and I have large moving boxes full of back-straining containers of Grendel brass I've shot since 2009.

I don't know why so many barrel makers in the after-market insist on using a .094" port on MLGS, because that is a recipe for exactly what you see in the OP.

Also, there is no such thing as a "Grendel II", "Type II" chamber, and the erroneous names you see for trying to identify bolt face depths and chambers.

The Grendel bolt design comes from the Beowulf bolt, which took notes from the extensive engineering on the Colt 7.62x39 that most companies never took the time to research.

Grendel bolts were always .136" recess because of the extractor lip, otherwise you get a thin lip that will shear off like most after-market x39 bolts do.

Black Hole Weaponry started making Grendel barrels many years after 6.5 Grendel had been on the market, only they were chambering based on a .124" bolt face depth and called that "Type I" for their in-house ID after customers asked them to do Grendel barrels with compatible chambers for Grendel bolts. After they started doing that, they called their Grendel-compatible .136" depth bolts "Type II". This gave customers the false impression that the .136" face depth was a later evolution of the 6.5 Grendel critical pressure containment components.

Because most manufacturers don't want to type everything I just did, they call their Grendel bolts "Type II", even though it's an obvious misnomer when you look at the timeline of how everything transpired.

When sourcing a Grendel barrel in the 16-18" lengths, make sure you get the correct gas port for the loads you will be shooting. If someone is selling a barrel with a gas port larger than .076" with 16" and 18" MLGS, you're going to need to choke that port off and still are likely to have problems.

There are even 16" CLGS barrels with huge ports, which makes zero sense. There was a great thread recently where a guy put a choke under his FSB to dial the insane gas down.

All the 18" Grendel barrels I have used with correct ports and have come from:

AA
Lilja
LaRue
Precision Firearms (PF has an Intermediate Length Gas System on 18" barrels that is between MLGS and RLGS)

JP makes great 18" RLGS Grendel barrels as well.

You can save yourself a lot of trouble, time, gas, and wasted ammo by sourcing barrels from companies that learned early-on how to configure chambers, gas ports, and gas system lengths.

In every case like this, I've seen the person trying to troubleshoot their DIY build spending more money than they would have even if they went with some of the higher-end options that are guaranteed bughole-shooters and reliable, usually because they were tricked by a low price point or recommended by someone who doesn't have more than a sample size or 2 assembling their own Grendels.

 

[jburtt]

You're way over gassed, which is causing your bolt to unlock early while under extreme pressure. This is causing the ring around the base of your brass.

I had the same issues when I first built my Grendel. Aero Precision upper and lower, Toolcraft bolt, 18" rifle length Criterion Barrel. One thing I noticed when I was putting it together was that the gas port seemed large. I started out with a standard carbine spring and H buffer. I had a basic $50 adjustable gas block, and I was getting 4 o'clock ejection at a 1/2 turn out from closed. I thought it was running pretty good until I put a can on it. I had to close the gas block almost all the way in order to get 3:00 ejection. I concluded myself that it was still overgassed, so I called Criterion and spoke to one of their technical guys who told me to put an h3 buffer in it. I did a little more research, and found that your higher pressure gas guns really like heavier buffers and extra power springs. I installed the Wolf XP carbine spring and an H3 buffer, which really smoothed out the rifles cycling. No more banged up brass. Now I can run the gas block a 1/2 turn out suppressed and two turns out without the can.

I've come to conclude in larger caliber AR's, most reported substantial improvements running an extra power spring and the heaviest buffer the bolt will reliably lock back on an empty mag when shooting your weakest loads. Mine runs like a Singer sewing machine now, and feels more like a 223.

A very good source of info and high quality parts is Clint over @ XH Buffers. Shoot him an email with the specs of your build and what you are experiencing and he will tell you which spring and buffer you should be using. I purchased multiple items from him and they all improved the cycling of multiple AR platform builds.

 

[longrange454]

I ran a ladder of 123's up to 27.6 of 8208XBR with no pressure signs.

What i found when running 8208. a chrono is your best friend. you'll see a spike in velocity before you see pressure signs.

 

[goatboy]

I’ve had same issue with a suppressed 6.5 grendel with rifle length gas system. you are over-gassed. Install an adjustable gas block and tune it.

Looking back, this thread goes to May 2020, please tell us that you worked the issue out.

 

[Greg Langelius *]

I went through a bunch of bad days with my new 6.5 Grendels, one is a 20" AR Stoner complete Upper, and the other is a self built 24" from AR Stoner components. Both have rifle length gas without any adjustment.

Back then they had Carbine collapsible stocks and buffers, and now they have basic A2 buffer tubes, buffers, and Choate E2 stocks, to facilitate bag riding. IMHO, these changes really had no appreciable effect on the issues encountered.

Just like you, I got lots of sooting, and lousy accuracy, but little to no pressure signs. Eventually, the 24" barrel's extractor went. Long story short, my loads were too hot, the sooting indicated there was too much pressure in the gas system too soon and the bolt was unlocking early. I had to replace the extractor, so I went with the JP Improved Grendel Extractor. I also changed from 120gr Gold Dot to 90gr TNT, I believe the overpressure, along with the added 4" of barrel generated enough dwell time that the extractor was overstressed and went bye bye.

Now, the rifle runs great and shoots tight with the 90 TNT and 31.5gr of IMR-8208XBR, at what I'm guessing is at or just below 3000fps. It was a pressure/dwell time issue. I think that driving a 120gr bullet at peak pressures in a 24" barrel simply subjects the system to too much stress for too long a time. I also think a 20" barrel is more appropriate for the 120, and that a carbine length gas system simply subjects the bolt to too much pressure too soon. Backing down my 120 load to 28.5gr of W-748 brought the system back into tune and shoots well also, but I'm using a rifle length gas system, too. I'm estimating 2300fps.

This is the barrel I'm using in 20", and the 24" one is the one where the extractor failed. I have a second 20" on standby in case I want to build another or just do away with the 24". I believe the modifications needed to convert the 223 bolt face to the Grendel dimensions weakens the extractor to the point where excessive loads cause breakage, and that the 6.5 Grendel is an exception where failure occurs well before pressure signs complain significantly. Literally, my extractor went at .1gr over published max. Where Midway sources its AR Stoner barrels is a good guess. I did a whole lot of looking around on the Internet, and came up with strong suspicions that at one time they were Bear Creek, and that later (now?) they are Liberty.

Right now I'm very happy with my Grendel;s, and I strongly suggest you back down the charges because I think you are probably on the verge of losing an extractor, just like I did. The 24" is now assigned for Varmints with the 90 TNT, and the 20" is for deer sized game or smaller with the 120 Gold Dot.

Greg