Not sure if this is the right forum, feel free to move it somewhere (not like you need my permission)

New Grendel rifle 20" Barrel, built by local friend with a FFL for dealer and manufacturing.  I'm sure he will be glad to check it out and fix anything, but I want to make sure I do my part before doing that, he makes me really good deals on everything (parts and shipping pretty much).

Took a few rounds I loaded with a published load less than max just to make sure my dies and other loading gear are producing something that will fire, cycle, eject, fit in the mags etc.  All that work fine.

However, strange bad grouping.  First two shots (100yds) were at 3'oclock, about 1/2 " away, less than 1" apart, great so far.

The next 3 were wild, 10 oc, 12oc, 2oc, all at least 4 or more inches away from the first 2.   Asumed my loads sucked shot 5 rounds of that Wolf Steel 100gr, and another 5 Hornady 123g Amax for comparison.

Some results, first 2 shots looking great, next 3 go crazy.  These are all manually single loaded, pushed in with the end of a plastic chamber flag before releasing the bolt, rifle on front and rear bags.

Nothing is obviously loose or moving, no strange noises, nothing feels wrong, function checks all good.

For a reality check I shot my old faithful Colt 5.56 HBAR with my usual 1.5MOA-ish groups consistently no problem.

Also took both rifles over to the steel side of the range and pinged 6" round steel consistently with both rifles, the HBAR grouping better.  I know, not a very scientific test, but just to demonstrate the Grendel does functionally work again.

My initiial thoght is something is unusually changing after when the barrel heats up a bit after the first two shots, but I've seen anything that drastic before.

A more experienced friend has suggested I shoot another 100 rounds through the Grendel just to make sure everything is broke in and settled down before I make any final judgements.  I have a total of about 120 rounds through it now.

Any other ideas? insults? mocking?

My first check would be the barrel nut torque. Can you give us a list of your build or what rifle including optics/sights?

Likely too tight or loose?



My invoice just list by description without brands, like "AR-15 Barrel, 6.5 Grendel".

I'm sure I can find out more details.

FWIW, I do know the upper is one of these: LeftSideChargingUpper

Yes, I requested it, although I know my builder friend has used these for other builds with good results.

..this reminds me of something else.  It is a bit finicky and requires releasing with the bolt release button in order to ensure full, solid bolt closure and avoiding click-no-boom due to light primer strikes.   Cycling the bolt by hand has very inconsistent results.

It also has a 3lb single stage trigger, one of these: DarkSkyTrigger

Everything else is standard as far as I know.

So, now I'm wondering if some combination of light torque on the barrel nut, with light trigger could cause a bad ignition or burn resulting in large pressure/velocity variation?  Just typing out loud..... but like I wrote earlier, no indications of any other problems.  Seems to me something like that would have more serious symptoms.

If it turns out to be either one I'll swap for a heavier trigger and different upper, I'm willing to lose the sexy side charging if necessary, and 4.5lb triggers are what I'm used to on an AR anyway.   We both enjoy tinkering and building so I'm not stressed about it.

I would need to know more about the details:

* We really need to know what barrel maker we are talking about, and what chamber they used.

* Was the upper receiver face lapped or square?

* What torque spec was used for the barrel nut?

* How does the barrel extension fit in the upper, tight, loose, was it bedded?

Different charge weights will print tight at 100yds, so it isn't that, and primer ignition isn't going to do that from a hammer strike variation.

I didn't get a good idea of what your groups were exactly from the OP.  Please post some pics.

Thanks for trying to help.  I'll have to collect that info, don't have it handy, won't have any target pics until after next range trip.

Do you have a thread protector on the barrel or a muzzle device?  My thread protector was loose and opened up the group really bad.  I took it off and groups went to .6"  That was with Black Hills ammo.  Now I just need to duplicate it.  BHW 264LBC 20" barrel BTW

The 6.5 Grendel generally is an inheriantly accurate round but it concerns me you were having trouble chambering your reloads.  Hornady A-Max or SST are very accurate factory ammo and should function in your rifle.  A box or two of factory Hornady should let you know if it is rifle/scope problem or ammo problem.

Irons or scope?  If you're using a scope, are you certain the mount is snug and that the scope itself is OK?  Just trying to cover all the bases.

I had the same results with Hornady A-Max.  Not sure what I wrote giving the impression it was having problems chambering only with my loads, it really wants the bolt slammed shut from the bolt release to shut all the way and fire not matter what ammo I used, handload, wolf, or Amax.

This was my 3rd range trip with this rifle, the first two did not exhibit this problem.

Yes, I have a thread protector and will be checking how it shoots without it, and if the problem goes away I will have to reconsider a flash hider or brake, or ??

That would make definitely make sense if it has now worked loose a bit......

Scope and mounts are securely mounted, checked with torque driver, leveled, this setup had great results on another rifle...

I'm now leaning with checking easiest thing first, in this case the thread protector.......to bad eating, sleeping, and work interfere with my range time...heh.......

Have you had it headspaced?  If you have a type 2 chamber and type 1 bolt it might allow you to slam the bolt home but be really tight.  If there is someone who can do that for you it could save a lot of frustration.  

Yes check the thread protector.  I was ready to send my barrel back until I found mine was not tight.   Blew through 100 rounds of ammo.  
Good luck

Also make sure your chamber is really clean.  If this only started happening on your 3rd trip it makes sense that you might have a little build up in there.

Heh, my other ARs all seem to shoot tighter groups the dirtier they are....I usually only clean their chambers and barrel every 200-300 rounds, I do wipe the bolt and lube it though...maybe this one is a special snowflake princess.

I seriously doubt that is the problem, these are extreme fliers, 3-4 MOA or more away from the first 2 shots.

Yea, that sounds just like the problem I had.  First 2 nice tight then anywhere from 2-4 inches off.  And no rhyme or reason as to the direction they are off.  Looked like I took a shotgun to the range.  I checked barrel nut, changed gas block, even bought a high priced BCG.  Nothing worked.  I was about to send it back and noticed the thread protector was loose.  I was so pissed I could not believe it.  Just eliminate that then move onto the more serious possibilities.

ETA;  I don't think the chamber being tight is your accuracy issue.  I noticed you were shooting wolf ammo.  If it is the steel case I have seen the lacquer stick in the chamber causing difficulty loading.  Just a thought.

You're right, the steel cased rounds did seem to have the problem sometimes even when the bolt was slammed shut, about 1 out of 20 rounds.  I'll keep that in mind.

UPDATE:

Unfortunately I confirmed today the problem is not with the thread protector being loose.  I removed it and the problem is still there.  My builder now has the rifle for inspection and evaluation to make sure everything is tight and in spec.

Glad it was eliminated.  Unfortunately still not a go.  Hope they get it taken care of.

Others have reported 4" groups with steel case.  Par for the course if that's the only ammo we're talking about.

Brass-cased Hornady should be around 1 MOA at worst in my experience.

Yesterday it actually got worse with the Hornady.

What are you using to clamp the gun when you torque the barrel nut?



I ask because I chased my tail for months trying to solve a similar problem once, only to find that the delrin clamshell clamp I was using was actually holding the nut off from bottoming out against the receiver.  I was torquing to 50 ft. lbs., but the nut was being torqued to the clamp, and not to the receiver, and so, I ended up with a loose barrel.



I relieved the face of the clamshell clamp with a dremel, torqued the barrel nut to spec, and the gun shot like a champ.

I'm letting the guy who built it for me do that for me.

What scope are you using. Something could of come loose in the scope and you've got a floating reticule.

This is possible, but I considered it unlikely because I checked and re-checked mounting screw torque all 3 range trips, and this scope has worked great on 3 other rifles, two of them .308.

The builder has told me he is checking into the gas block and the way the barrel nut, extension, and upper all fit together.  I told him to feel free to shoot it or have anyone else shoot it with known good scopes enough to verify the results.

Not directly related to the accuracy issue but I will address the issue of having to drop the bolt to completely chamber the round.

I understand correctly these are all reloads.  I had the same issue with some of my first reloads using the Hornady dies.  After comparing my reloads to factory ammunition I found that the Hornady dies were not bumping the shoulder back.  I was able to bump the should back just enough by adjusting the die down as much as possible and still allow the press to cam over.  The dies really need a little taken off the bottom to allow proper adjustment to bump the shoulder back properly.  I don't know if this is inherent to the Hornady dies or just my set.  BTW I have a AA barrel and bolt.

Try chambering some factory Hornady ammunition and see is they chamber better.

No, happened with Wolf poly, and Hornady factory rounds, never happened with my loads.

My best groups have been shot using Sierra's 107 grain Match Kings and 26.0 grains of VihtaVuori N530.

Only testing two brands of ammo (with one being junk from the get go) really doesn't tell you much.

More testing is in order.

UPDATE:

Summary : problem fixed, last results with 5 shot groups, 3 in "bug-hole", other 2 in within my range of sukage.  Promising loads:  28.0 Varget and 27.5 IMR4895 ... seated to whatever I measure the Hornady amax ammo at.  The only other powder I have is RL-15, and it wasn't quite so good.

Builder found evidence of gas leaks.  Replaced gas block, polished it up some and where it fit on the barrel first, also polished up where the upper and barrel extension fit.

We don't know for sure what the problem was, but it is much better now.  Last shot of the day hit a 3" gong at 300yds with it, earlier hitting the 6" gong seemed strangely easy...

Awesome, glad you got it shooting right!

For a builder to need to replace a gas block and polish things, no. Polishing parts opens up the amount of slack between them. A gas block needs to be tight, the barrel extension as closely fitting as possible to the upper. Your "licensed FFL" builder is giving you indications that his skill level isn't all that.  

As for "barrel nut torque" the spec is minimum 30 foot pounds to a maximum of 85 foot pounds, that's a 55 pound leeway.  There is no published table of barrel nut torques that is guaranteed to improve accuracy. The rifling has far more influence.

Enjoy the gun, glad it's fixed.

Ive also never been happy with my grendel.   It is a 24" er shaw barrel.  Not the best I know.  I get consistent 1.25 moa.  Ive tried handloads and factory.   It is one of their ealier barrels that is the grendel chamber not the grendel ll.  It feeds and functions fine.  Anyone have this barrel with any suggestions

"Your "licensed FFL" builder is giving you indications that his skill level isn't all that. "

This is true and acceptable to me.  He is just starting out, and only building these for a few friends who are experience shooters to evaluate.  He sells to me at a cost that is less than I could build it purchasing everything myself.    
I think he buys in enough bulk to get a dealer/builder discount of some kind.  His shop has tools I can only dream of.  

My level of experience and skill, especially regarding fitting barrels or gas blocks is less than his.  In fact it is 0, I've never done it.  None of his previous builds have had any reported problems or required any re-work afaik.