308 with accuracy/consistency issues

[Samsquanch]

I'm hoping someone more knowledgeable can help me decide my next move. I have a home built ar10 that is very inconsistent. Major parts are:

Aero m5e1 builder kit
Aero BCG
Rainier Ultramatch mod2 18" 1:10 308 barrel
Superlative arms gas block
Geissele ssae
Jp buffer system

When trying to work up a load for it I noticed it is wildly inconsistent. A few different times I found loads that shot really well on the initial test groups, but then would open up like crazy for no apparent reason. The most extreme example was a load that shot multiple .25 moa groups, and the next day when I set up to chrono it the same recipe measured over 3 inches on the first group, and never dropped below an inch again on further testing. I lapped the receiver and bedded the barrel. I double checked all torque values. I also used go/no go gauges to check headspace. I tried shooting it dirty as well as after a deep clean. After switching powders and projectiles multiple times, I never have found a way to make it consistent, whether good or bad. I tried shooting a box of berger 175s to see what it would do with factory ammo, and it shot an average over 10 rounds of almost 4 inches.

So the spot I'm in now is I'm wondering how to fix this rifle. Maybe Rainier barrels just suck, and I need to rebarrel it. I'm more inclined to think something else is contributing to this though. I've had several people tell me I'm lucky if I get an ar10 under 2 inches, but I've seen enough examples to the contrary to think this isn't the maximum potential of the rifle.

One thing that may or may not be related is a weird symptom I'm seeing on the base of some of my cases. I see what looks like a smear of brass near the primer. Something is gouging into it and pushing the brass into a little bump that I can flake off with my fingernail. The fact that it only happens to some of the brass and not all tells me something inconsistent is happening that may be related in some way to my accuracy issues. So do I just need to get in line for a high end custom barrel or is there something else to look at? Thanks!

 

[hlee]

Anyone that says 2 moa is the limit for an AR10 needs to be excised from your life like the cancer that they are. Beyond that, start by eliminating your reloads from the mix. While it is a terrible time to be sourcing ammunition, federal gold medal match is the gold standard and should shoot better than moa in a capable rifle. Assuming that doesn’t shoot well, check all the torque values. Loose scope base. Loose rings. Loose barrel nut. Does the carrier key interfere with the gas tube during its travel, is there any hang up in the bcg as it moves within the upper? Is the gas tube or gas block hitting the handguard somewhere?

 

[hafejd30]

Sounds like your getting an ejector swipe. Due to over pressure. The mark with the dimple of brass you referred to

My AR 10 although accurate (Rock River LAR8) would work as a bullet puller when the action cycled. Moving the bullet forward in the case .010-.020” when the BCG slammed shut.

Possibly could be doing the same with yours. Which would cause inconsistency and pressure spikes. No matter how much I crimped the case I still got .002-.005” of movement.

Measure a round. Let the BCG slam shut loading the round from the mag. Remove the round and remeasure the length

Otherwise give the gun a bare metal cleaning and start over. Maybe the barrel fouled out quick

 

[Feniks Technologies]

Are you an experienced gas gun shooter?

90% or more issues with AR platforms is user error on fundamentals. Gas guns flaws will show on paper much more than bolt guns. Several reasons for this. Being a good bolt gun shooter does not translate into gas guns unless you’re really hammering fundamentals.

If you’re not an experienced gas gunner who knows you can drive the rifle properly, I’d start there. If that is indeed the issue, nothing you change with the rifle will improve performance.

 

[hafejd30]

As far as the accuracy is concerned a modern AR with a decent barrel should get under 1 moa pretty easily with good ammo.

My LAR8 would shoot 1/2-3/4 with FGMM 168 grain and about 1 moa with 165 hornady superformance

My old Armalite with a 16” pencil barrel and non free floating barrel was a 2 moa gun

 

[hlee]

Amd my dpms lr308 (24” fluted bull barrel) was moa or better with Hornady match and FGMM, right out of the box, with a noob behind the gun…

 

[Samsquanch]

I tried some 168 gmm when I first built the gun. I didn't know I had a problem back then so I didn't pay much attention to exactly what it shot, but I know it was over an inch. I don't recall seeing any wild flyers, but it wasn't sub moa. I remember thinking it was odd that round didn't group better when most guns seem to like it.

I tried stripping the copper and giving the barrel and chamber a thorough cleaning. Didn't seem to have any impact on the consistency.

I chambered one of my hand loads from the mag and measured it when I was playing with neck tension. No change in overall length.

As far as shooting ability, I'm no expert, but I can shoot my other ars consistently under an inch, and my bolt gun under .5 if I'm doing well. I'm shooting off of bags trying to contact the rifle as little as possible. Nothing fancy as far as my process there. Seems to work for bolt guns and small frame ars. I'm new to ar10s though. But 3 inch+ groups shouldn't happen unless I sneeze every time I'm pulling the trigger.

As far as my expectations, I would just like to be under an inch and consistent. I'm not a competition shooter with extreme expectations.

 

[hlee]

Post a pic of the “trouble spot” on the brass.

 

[Samsquanch]

20211202-193135 hosted at ImgBB

Image 20211202-193135 hosted in ImgBB

ibb.co

 

[Clayman]

I tried some 168 gmm when I first built the gun. I didn't know I had a problem back then so I didn't pay much attention to exactly what it shot, but I know it was over an inch. I don't recall seeing any wild flyers, but it wasn't sub moa. I remember thinking it was odd that round didn't group better when most guns seem to like it.

I tried stripping the copper and giving the barrel and chamber a thorough cleaning. Didn't seem to have any impact on the consistency.

I chambered one of my hand loads from the mag and measured it when I was playing with neck tension. No change in overall length.

As far as shooting ability, I'm no expert, but I can shoot my other ars consistently under an inch, and my bolt gun under .5 if I'm doing well. I'm shooting off of bags trying to contact the rifle as little as possible. Nothing fancy as far as my process there. Seems to work for bolt guns and small frame ars. I'm new to ar10s though. But 3 inch+ groups shouldn't happen unless I sneeze every time I'm pulling the trigger.

As far as my expectations, I would just like to be under an inch and consistent. I'm not a competition shooter with extreme expectations.

Have you checked the optic/mount?

Could always swap it out with another rifles optic to rule it out.

On your brass, looks like pressure sign to me. Ejector mark.

 

[Samsquanch]

I put the optic on another rifle and it shot normally.

Good to know that's a pressure sign. I've never seen that before on any brass.

 

[hlee]

Ejector swipe. Flat primer- not super flat, but flattened. And a bit of firing pin cratering.

 

[whatsupdoc]

Are you an experienced gas gun shooter?

90% or more issues with AR platforms is user error on fundamentals. Gas guns flaws will show on paper much more than bolt guns. Several reasons for this. Being a good bolt gun shooter does not translate into gas guns unless you’re really hammering fundamentals.

If you’re not an experienced gas gunner who knows you can drive the rifle properly, I’d start there. If that is indeed the issue, nothing you change with the rifle will improve performance.

^^^^^^^
Start with this right here.
Have a good AR10 shooter test the rifle. I have to concentrate like a MoFo when I shoot
a AR10 or it will immediately let me know that I made a mistake somewhere in the shot.

It will seem like the rifle has an issue but most times the rifle is not at fault.

 

[mtrmn]

20211202-193135 hosted at ImgBB

Image 20211202-193135 hosted in ImgBB

ibb.co

Looking at your pic. That appears to be an ejector swipe. From an ejector with sharp edges. I've been told that the problem with AR's grouping sporadically depending on which side of the magazine the round feeds from could very well be due to a sharp edged ejector. The cure, so I was told, is to remove the ejector from the bolt, chuck it in a drill and hold it at a 45* angle against a sharpening stone to round off the sharp edge. That sharp edge gouging into the brass during the feed cycle somehow provides enough resistance to affect how the round chambers depending on which side of the mag the round feeds from. Worth a try, you ain't gonna hurt it as long as you just round off the edge a little, and if it works you fixed it for free.

 

[hafejd30]

What optic?

 

[Samsquanch]

Looking at your pic. That appears to be an ejector swipe. From an ejector with sharp edges. I've been told that the problem with AR's grouping sporadically depending on which side of the magazine the round feeds from could very well be due to a sharp edged ejector.

Would there be a visible case sign to look for or would I just need to do it and see what happens?

^^^^^^^
Start with this right here.
Have a good AR10 shooter test the rifle. I have to concentrate like a MoFo when I shoot
a AR10 or it will immediately let me know that I made a mistake somewhere in the shot.

It will seem like the rifle has an issue but most times the rifle is not at fault.

Can you elaborate on this some? What is fundamentally different about shooting an ar10 vs a bolt gun or ar15?

 

[Samsquanch]

What optic?

Bushnell DMR II 3.5-21

 

[mtrmn]

Would there be a visible case sign to look for or would I just need to do it and see what happens?


Can you elaborate on this some? What is fundamentally different about shooting an ar10 vs a bolt gun or ar15?

Look at the elongated ejector swipe in your pic. See the raised sliver of brass in the middle of the mark? That was made by the brass sliding across the sharp edge of the flat-faced ejector. A rounded edge won't gouge out the brass to create that raised portion.

 

[Samsquanch]

Understood. I'll take a look at the ejector tomorrow.

 

[mtrmn]

Understood. I'll take a look at the ejector tomorrow.

Go here: https://forum.308ar.com/topic/21414-can-a-gunsmith-diagnose-accuracy-issues/
Read the last 3 posts in that thread. Wouldn't hurt to read the whole thing it's pretty short.

ETA: here's a KAK ejector that's completely rounded. You don't have to go to that extreme, but it goes to show you that it would be hard for you to over-do it. https://www.kakindustry.com/lr-308-parts/308-bcg-and-spare-parts/kak-bcg-ejector-308

 

[impactaddict]

I'm shooting off of bags trying to contact the rifle as little as possible.

Could be your loads - maybe not - but I think part of the problem is here. After I read the following article, my gas gun shooting got significantly better. The 15lb dumbbell trick is a mindblower to me. I’m no expert but I’d suggest reading that and trying again.

Mechanics of the Firing Hand

Sniper's Hide, Precision Rifle Instruction, Training Shooters, Acccuracy International Rifle Bipod, Fundamentals of Marksmanship Mechanics

www.snipershide.com

 

[whatsupdoc]

Would there be a visible case sign to look for or would I just need to do it and see what happens?


Can you elaborate on this some? What is fundamentally different about shooting an ar10 vs a bolt gun or ar15?

Locktime and gas flowing through the system while the bullet is in the barrel makes your setup, hold and follow through
much more important. I can shoot a bolt rifle loosely without any cheekweld no pulling the rifle into my shoulder as long as my
trigger pull is good I can make good shots. I call this my lazy shooting no fuss no muss.

With an AR10 I have to make sure to take the slack out of the bipod pull the rifle into my shoulder, make sure
my shooting hand is in the correct position trigger finger correctly on the trigger and while doing this
find the target while not changing anything and make micro adjustments with the rear bag hand while squeezing the trigger.

 

[psmith23]

I've never had good luck shooting ar10's anything close to free recoil. Best is when I give a decent preload to the bipod, really focus on fundamentals and pretend to be a sack of dead meat behind the rifle when it goes off. Make sure you are holding the trigger all the way to the rear for the whole firing sequence and are able to hear the reset click when you release. Also as said, check for loose mounts and rings, try different optic etc. If you can try a different shooter that would be great. Case in point my buddy last weekend said his AI shot like shit with my ammo. Looking downrange it looked a little over a moa through the spotter. I said really....since it was basically 43 grains of varget with a 175 @ 2.80 which basically shoots great in all of my rifles. I tried 5 rounds, all touching somewhere around .5 moa. Fundamentals are much more important with a large gasser. With that said since you shot small groups before, try to check for anything loose. Fyi I've had loose muzzle brakes really effect accuracy.

 

[bfoosh006]

IMHO... you may have already hit on the answer.
The AR design has so many moving parts, all those parts "need" to go back to the same consistent position to fire precisely sized groups.
That comment also includes your function in that equation.
Use Steady, very consistent bench techniques. Lock your body, inline with the recoil impulse and fire a group... don't fidget at ALL.
....don't even readjust your check weld on the stock. Hold that solid position throughout the group.
I tend to hold the stock firmly into my shoulder, but I don't lean into the stock to hard.... just enough to hold the handguard in the same position on the front bag.

For that matter... I didn't see if your were using a bipod or bag or whatever... try different setups. I shoot better off a good front bag opposed to a Bipod.

Recheck that every part is snug and to a reasonable torque spec.

Make sure you have no Gas Block to handguard contact.... ( not even close to the barrel making contact with anything )... make sure the gas tube freely enters the gas key. No binding or being off center. ANY less then perfect fit can cause wild groups.

That ejector smear, does show a less then ideal smoothness... and the case head looks like you are maybe getting abrupt extraction... so polish that ejector face / edge.

I am not 100% sure your Adj. GB is adjusted properly ( abrupt extraction ) ... so you might try the load a single mag fed round and adj. until consistent lock back trick.

And shooting large frames requires a whole different style than shooting a AR15 ... I have to use a lot more follow through at the bench.
And the larger recoil impulse ( moving parts ) will "amplify" any lack of skill you ( or I ) could ignore with a AR15 / Bolt gun at the bench.

What are the specifics to your ammo tried and handloads ? Is it possible your barrel hasn't "broken in" yet ? .. or maybe the barrel doesn't like your ammo choices ?

To be completely honest... there can be all sorts of possible causes to your unsatisfactory precision. So, please try to answer any questions as best as possible.

 

[Samsquanch]

I did some additional testing today. I shot 8 rounds of berger 175s loading each round individually to control for the ejector issue mentioned above and it shot right at 1 inch over 8 shots. Picture attached. I only marked 6 shots because the other two were in the same holes as 2 of the other shots.

Next, myself and another shooter shot two 5 round groups each of 168 gmm and it shot those between 1.5 and 2 moa. No significant differences between our groups.

I took the muzzle device off and shot it that way with no significant change (big poi shift but accuracy was similar).

Gas setting is as minimal as it can be while retaining proper bolt lock. Nothing is touching the barrel or rail and the gas tube is aligned properly.

Round count should be around 400 in total now, and I always follow manufacturer instructions for barrel break in.

For my hand loads, I'm using Lapua brass, cci 200s, and I've tried h4895 and n540 powder, as well as berger 168s, berger 175s, and match king 168s. I've played with overall length and neck tension quite a bit between those.

I'm going to try to borrow a different bolt for testing but short of that, I'm thinking the barrel just isn't a shooter at this point. Good to know because I was considering Rainier for another build, but I'll go a different direction at this point just to be safe.

 

[BCP]

Is that one six shot group that winds up looking like two groups? Maybe you aren't getting a good cheekweld and your head is moving around.

Also 41.5 H4895, LC brass, 168 or 175 SMK and 2.8".

 

[Samsquanch]

Thats one 8 shot group. I had to come off the gun each time to load individually for that test. There are actually 5 rounds in the bottom 3. It shot the 3 high shots first, then the group moved down for the last 5.

 

[BuildingConceptsllc]

I tried some 168 gmm when I first built the gun. I didn't know I had a problem back then so I didn't pay much attention to exactly what it shot, but I know it was over an inch. I don't recall seeing any wild flyers, but it wasn't sub moa. I remember thinking it was odd that round didn't group better when most guns seem to like it.

I tried stripping the copper and giving the barrel and chamber a thorough cleaning. Didn't seem to have any impact on the consistency.

I chambered one of my hand loads from the mag and measured it when I was playing with neck tension. No change in overall length.

As far as shooting ability, I'm no expert, but I can shoot my other ars consistently under an inch, and my bolt gun under .5 if I'm doing well. I'm shooting off of bags trying to contact the rifle as little as possible. Nothing fancy as far as my process there. Seems to work for bolt guns and small frame ars. I'm new to ar10s though. But 3 inch+ groups shouldn't happen unless I sneeze every time I'm pulling the trigger.

As far as my expectations, I would just like to be under an inch and consistent. I'm not a competition shooter with extreme expectations.

Think you found the issue "I'm new to AR10's" , but.... I wouldn't be surprised to learn that a new quality barrel had you shooting better either. AR10's are more difficult to shoot accurately than small frame ar's. I bet if you practice good fundamentals and shoot your group slowly focusing on those you will be at 1moa but that barrel may not do better than that. Jmo

 

[Samsquanch]

I can borrow a 6.5 ar10 from a friend that is well proven to test my shooting ability with it. That might be a simple way to see if I'm influencing something. I was hoping the other shooter from today would see different results, since he's more experienced than me, and also "gets on the gun" much more than I typically do, but our groups were similar unfortunately.

 

[st1650]

What mount ? I’ve seen plenty of people unable to find why their gasser was inconsistent only to find out they were using Larue and ADM vertical split mount. Probably the easiest way to remove the biggest variable would be to move that optic + mount combo on a proven rifle and see if you get the same inconsistency. I was having a similar issue that would drive me crazy with a Vortex Razor LHT HD where I would do a 1/2” group then the next day, same load, would do 4”. Ended up being the scope.

Also, try establishing a baseline with something proven like FGMM 175, it you can’t consistently do better than 1 moa 5 rounds then there’s definitely something wrong with your optic or rifle.

 

[Samsquanch]

I'm using an adm delta mount. I moved the optic to a small frame AR to test and it was fine, but I didn't try a different scope on the 308.

 

[hlee]

What mount ? I’ve seen plenty of people unable to find why their gasser was inconsistent only to find out they were using Larue and ADM vertical split mount. Probably the easiest way to remove the biggest variable would be to move that optic + mount combo on a proven rifle and see if you get the same inconsistency. I was having a similar issue that would drive me crazy with a Vortex Razor LHT HD where I would do a 1/2” group then the next day, same load, would do 4”. Ended up being the scope.

Also, try establishing a baseline with something proven like FGMM 175, it you can’t consistently do better than 1 moa 5 rounds then there’s definitely something wrong with your optic or rifle.

Or shooter…

 

[mtrmn]

Thats one 8 shot group. I had to come off the gun each time to load individually for that test. There are actually 5 rounds in the bottom 3. It shot the 3 high shots first, then the group moved down for the last 5.

If you started this group with a cold barrel, that could account for the first 3 going high and the group moving down as the barrel heated up. IF THAT'S THE CASE, then it looks to me like your single-loading experiment actually worked and the "depending on which side of the mag the round came from" scenario could actually be your problem.(?) If you look at that picture as 2 different groups (cold bore vs hot) then they are both pretty respectable IMO.
I'm just trying to prevent something being overlooked here. Ammo and components aren't cheap and you can go through a lot of it chasing ghosts.

 

[BCP]

Eh I bet barrel is fine, could be something loose though. How are you testing? Bags and bench? Bipod? Prone? I do my "serious" accuracy testing with a lead sled and lots of sandbags.

 

[Samsquanch]

I made the load BCP posted above to see what it would do just for something to try. 4 shots at 2.71 moa.

I let a friend shoot his hand loads that work well in his rifle out of mine (berger 185s), and 3 shots measured 2.47 moa on that one. We single loaded everything as I did last time on both tests.

I'm going to see if any local gunsmiths can look it over and suggest anything. Short of that I'm down to either rebarreling or selling it and buying a factory built gun. I think ill be spinning my wheels chasing different loads from here. A barrel with a matching bolt and an accuracy guarantee is probably where I should have gone from the beginning on this build.

 

[BuildingConceptsllc]

Can you post pics of the gun? Might help. Also pics of the barrel a d crown. Something just isn't right here.

 

[Samsquanch]

I'll take some when I can. I looked at the crown when I pulled the muzzle device off and it looked normal. Lots of build up at the muzzle, but nothing changed when I cleaned it up and shot it with the comp removed entirely. Just a big poi shift but no accuracy improvement. The groups today were shot with no muzzle device.

 

[Samsquanch]

Eh I bet barrel is fine, could be something loose though. How are you testing? Bags and bench? Bipod? Prone? I do my "serious" accuracy testing with a lead sled and lots of sandbags.

Bags and bench mostly, but I tried a bipod off the bench for a few groups.

 

[psmith23]

If you got a handload with Berger's to go about an inch and the other standard loads are 1.5-2" then at this point I'm going to say that's all she's got capt'n. Friend at the club a few years back ended up wasting half a summer on a prefit barrel that was almost 1 moa. Ended up swapping it out and down to .5 moa consistently with various loads. At this point I'd rebarrel. I've had great results with a prefit krieger m110 profile and a bartlein that Craddock made for me but can't speak to anything else.

 

[BuildingConceptsllc]

If you got a handload with Berger's to go about an inch and the other standard loads are 1.5-2" then at this point I'm going to say that's all she's got capt'n. Friend at the club a few years back ended up wasting half a summer on a prefit barrel that was almost 1 moa. Ended up swapping it out and down to .5 moa consistently with various loads. At this point I'd rebarrel. I've had great results with a prefit krieger m110 profile and a bartlein that Craddock made for me but can't speak to anything else.

Yeah, that's what I'm thinking. If OP is absolutely sure everything is put together right with the barrel and the nut is secure, scope, rings, mount, rail... all check out as solid and not moving. ..... (Which it appears that's the case), then the ammo tells the tale. It's the barrel.


OP, have you talked to Ranier about this? Maybe they will send you another barrel???

 

[Samsquanch]

I'm thinking I'll see if I can send the barrel back to have them determine if it's out of spec. Funny enough, talking to a friend that just built 3 ar10s in different calibers, the only one that isn't performing as expected out of the gate has a Rainier barrel on it. It doesn't sound like it's as bad as mine, but it won't group any of the factory match ammo well so far. It's anecdotal, but I thought it was interesting.

 

[psmith23]

It will be interesting to see how the barrel comes out after being bedded. My two uppers have all been thermal fit (heat upper/freeze barrel) but never dealt with any bedding or adhesive.

 

[Makinchips208]

Is it possible for side play between upper and lower receivers, or sloppy takedown pins to cause this?

 

[Billiam1211]

More likely than not, it’s like a bunch of the comments above that it’s just adjusting technique and hammering the fundamentals. I shoot a lot of .308 gas gun in matches and it really tests the shooter. I built a precision 308 gasser with a Bartlein barrel and all JP parts. Tried 3 or 4 barrels, bedded my barrel, squared the receiver face… did all that stuff and it still came down to my fundamentals. When I’m on top of my game I can clear KYLs and print 1/2 minute groups. When I’m off…. it ain’t pretty. The last time at the range, I had to shoot some groups and decided to try an experiment. I varied bipod load pressure and shooting hand grip pressure pulling the stock into the shoulder while keeping the other fundamentals the same - really exaggerating follow throw (press, break, freeze). Results were insane: shots where I didn’t load and had weak grip printed groups that were 1.5-2 MOA. As I increased bipod load and shooting hand grip, groups tightened up to around 0.6 MOA. I also noticed recovery and recoil management improved and that crosshairs would immediately settle right back on target. Before you start ripping it apart and tinkering I’d try spending 50 rounds on experimenting with technique

 

[Screwgunner]

I got some 168 gr. Hornady a max match ammo and my ar loves it . Took me 1 year to figure out the load 42 grains 3031 2700 fps

 

[r.tenorio671]

20211202-193135 hosted at ImgBB

Image 20211202-193135 hosted in ImgBB

ibb.co

....pretty significant ejector smear and very flat prime there... over-pressured and most likely timing issue, just my $0.02...

 

[JohnCarter17]

Looking at your pic. That appears to be an ejector swipe. From an ejector with sharp edges. I've been told that the problem with AR's grouping sporadically depending on which side of the magazine the round feeds from could very well be due to a sharp edged ejector. The cure, so I was told, is to remove the ejector from the bolt, chuck it in a drill and hold it at a 45* angle against a sharpening stone to round off the sharp edge. That sharp edge gouging into the brass during the feed cycle somehow provides enough resistance to affect how the round chambers depending on which side of the mag the round feeds from. Worth a try, you ain't gonna hurt it as long as you just round off the edge a little, and if it works you fixed it for free.

I basically remember hearing the same thing, on the Grendel forums, about the need to eliminate the sharp edge on the ejector. I also remember what a pain it was to do this without a tool to hold the bolt. I was doing it then for reliability to remove the unneeded doughnut.

So thanks for motivating me to order the tool from Brownells, and I will do some PM on my 223 and 264 bolts.

 

[eastexsteve]

I've had several people tell me I'm lucky if I get an ar10 under 2 inches, but I've seen enough examples to the contrary to think this isn't the maximum potential of the rifle.

Maybe THEY have a problem getting an ar10 under 2 inches. But, I bet it's not the guns they are shooting. In reading this thread, it appears that you are juggling a lot of variables here. (Don't forget, the shooter is one of the variables.) And yes, ar10s are a different animal when it comes to shooting them accurately. I'd start from scratch. Make sure everything is tight. Ditch the supressor. Forget guessing your reloads. Call the barrel maker and ask them what they use to test their barrels. I'll bet it isn't your reloads. Otherwise, start with 50 rounds or so of good factory target loads. They don't have to be FGMM, but something that has a reputation of being consistent. Hell, I've got one ar10 that shoots PPU 175s better than FGMM 168s. If your first choice doesn't produce results, go to another brand or weight. (I wouldn't go over 175gr though) But, take your time and don't try to rush the process.

All other things considered equal, ar10s are little weird, but there isn't really any magic to them. I have two lightweight factory ar10 builds with 18" 308 Faxxon pencil barrels that are a few years old now. They have many hundreds of rounds through them. One is more beat up than the other because I used it in heavy irons 3-gun matches. They are both 1,000 yard shooters. They have a little play between the upper/lower. The barrels are on tight. Scopes, rings, mounts, ditto. No gas, feed, or ejection problems. As long as I do my part, they are MOA or better guns on a good day. But, If I don't pay attention to what I'm doing when I pull the trigger, I WILL get a flyer.

 

[Reaper77]

Way late on this thread, but I am having the same exact issue with my AR-10. Have tried at least 8 different brands of ammo across a plethora of weights. Best groups are right around 3.5" at 100 yards, but most are around 6-8" groups. I've checked all tolerances, and torque on the barrel nut; nothing out of spec. Have tried two different scopes to make sure that wasn't the problem as well.

 

[bfoosh006]

Way late on this thread, but I am having the same exact issue with my AR-10. Have tried at least 8 different brands of ammo across a plethora of weights. Best groups are right around 3.5" at 100 yards, but most are around 6-8" groups. I've checked all tolerances, and torque on the barrel nut; nothing out of spec. Have tried two different scopes to make sure that wasn't the problem as well.

Specific parts used , or brand of Rifle ?
Ammo tried ?