I recently built a large frame AR and am using a RanierArms ultramatch 308 stainless barrel (18”).

Barrel is properly installed and torqued to factory specs.  All other equipment is premium and tight (no loose scope mount or anything).  I am a veteran shooter and shoot mid to long distance often.  

Shooting from a bench with a bipod and rear bag (and 18x scope for clear visibility) with regular factory non match ammo I am getting 3.3-3.5MOA measured on a non windy day shooting several 5 round groups at 87 and 200 yards.

Shooting again as above but with factory M118LR match ammo (ouch was that expensive in today’s market), I am getting 1.92-2.29 MOA with potentially as bad as 3 MOA if I don’t discount one that could have been effected by wind.

I have fired approximately 75 rounds through the barrel and cleaned it several times in between.

I believe that these groups are no where near where they should be, but wanted to check in here just in case my expectations are way off.  I don’t know if I have a bad barrel or if that is about what I should expect.

It has been a while since I have shot for groups with my 5.56 rifles, but I recall getting around MOA or just under when using match ammo.  I would not have expected 308 to be so much worse.

What about the bolt? I've changed bolts and have had shifts in accuracy. Have had best luck with JP bolts. Could be the barrel, but I would rule everything else thing out first since its the most intrusive part to swap out.

I've had good luck with fire lapping. The bullets are rolled between two hardened steel plates to force an abrasive into them. Progressively finer grits are used. I have never had a decrease in accuracy  when using this method.

My brother and i both built 5.56 rifles with stainless steel barrels nothing fancy mine was a PSA barrel and his was a RRA.
Both shot terrible with any ammo.
We've both since moved them on.

They have a 90 day return policy - I'd just send it back.

I played this game for a bit on a build for a friend, using a barrel that was arfcom approved at the time, and it was terrible.  My friend decided to get a Dan Lilja barrel.  It was an incredible immediate difference and shot hunting soft points really well, and Gold Medal Match into clover leafs.  Life is short.  Get a great match barrel.

I'd run a cotton down the bore to feel the gas hole for any burrs.

I have in the past re-torqued large frame AR receivers and improved the groups. You might also re-torque the muzzle brake if it has one. Check the crown to see if there's fowling building to one side or the other. Check the gas tube to see if it's rubbing the impingement port of the bolt carrier. Trouble shooting large frame AR's is a pita due to the differences in tolerances. You might also try hand lapping with some Kroll oil and JP paste. But as the above posters have mentioned shooting a large frame AR requires a bit more fallow through and trigger control. That's where the adjustable gas block shines in taming some recoil pulse. I've even had some magazines effect my groups. How does it do when you single load and allow for lock back?

It’s not uncommon to have a single round cause a group to really expand. From my experience, this is usually the first or last round from the magazine.

If this is your first large frame AR, you may want to let someone else shoot it before blaming the build/barrel. I’ve seen guys that were good bolt gun shooters look like a novice when handed a large frame AR. They would shoot 2.5-3” groups and then I could use the same rifle/ammo and get sub moa consistently. They are still good shooters, but needed to understand the difference in proper technique with a large frame gas gun. After changing up their technique, they started printing groups that were half the size of their previous groups. In short, a LF gas gun will exploit any flaws in your shooting form.

As others have mentioned, if you can get some FGMM 168’s, they should be great for determining the potential of the build/barrel.

I have 16in Rainier Match SS barrel with excellent accuracy.  But it does have some loads that crazy open up the groups.

I would try some more loadings and contact Rainier for some help.  They have always been great to deal with.

I would expect 1" groups or less depending on a lot of things. I usually try to focus on building a good position with natural point of aim, slowly squeezing the trigger and keeping it held back as I analyze where I've impacted on the target.

Group shape will tell you a lot of things as well, it would be helpful to have a picture of your groups. Lastly, to mitigate double recoil impulse, I will load one round into the magazine at a time and focus on all these things every shot. I know you've stated you're a veteran shooter, so all of this may be redundant.

I am going to second a lot of advice here and suggest different ammo and/or a call to Rainier. They are a good outfit. I have a Ultramatch barrel of theirs on my 6.5 and it is great.

My Grendel which is usually produces tight group will open up with a certain brand. The group will open up so much, I dont get that ammo anymore.

Thanks all so far for the advice.  To answer a few questions:

—Has adjustable gas block, adjusted for running suppressed.
—Maybe should have mentioned but all shots have been suppressed
—recoil impulse feels really good actually.  Stronger than 5.56 but it is a solid and consistent push, not too sharp.
—cleaning has been just regular gun cleaner (forget the brand) down the bore and Run a snake several times.  
—Bolt is from a complete BCG by CMC, Their enhanced bcg.
—I tried prone, off a bag, and a few other positions but bipod on the bench with a rear bag was the most stable.  I feel very good about the positional stability for these and especially seeing such poor results at so close range...87 meter (not an arfcom joke, actual distance)

What to do next:
—I will source some FGMM and run that.
—I do have some J-B bore cleaner paste I can run through it.
—I will Retorque the  barrel just in case.

I don’t have a good way to host pictures so that is always tricky,  groups tend to be relatively even spread slightly squashed ovalloid (wider than taller) but still relatively round shaped (not pancake).  Some of the groups have one round that is slightly further away that the others but never by more than 50%.

Should I adjust the gas to be “off” and just hand cycle to shoot groups so recoil impulse can’t be a factor? Would that provide any potential improvement?  At least to provide some kind of benchmark “best possible grouping”.  I can do single loading and remove the mag each time to exclude that variable.

It will cost more money in ammo but I can do 10 shot groups vs 5 to make sure I have consistent data.

I’ll give it all one more go and if I don’t see an improvement I will return the barrel I guess.

Ok.  I knew I was going to catch some flak for that cleaning.  I do have some rods and slip 2000 carbon killer if that is close enough, but probably need to buy some 7.62 sized tool ends.  

Regarding the images, I only see attach emote, meme, or deal.  Probably have to be a paying member to attach images.

I didn't see details of your build.

Is it free-floated?

What kind of trigger do you have?

Are you getting baffle or end-cap grazing?

Military surplus M118 LR is just meh.

Again, bags just might be better than a bipod for resting.

You should really try to source proper cleaning equipment and solvents if you're trying to break-in and season the bore.

With ammo as expensive as it is I would contact Rainier sooner rather than later. At least touch base with them before wasting a bunch more time and money.

I performed a more substantive barrel clean and am heading back out today.  I am brining along a SPR style 5.56 that I know performs well to get some baseline groups to compare my position and and wind situation.

I will report back in, but my main question of do I have a bad barrel or not has somehow turned into “if you don’t clean a barrel perfectly you will have terrible accuracy”.  I don’t doubt that for benchrest shooters and the like that not cleaning a barrel will lead to larger groups.  But by how much?  Is my 3 MOA barrel going to suddenly become a 1MOA barrel because I didn’t properly clean it after only 30 rounds?  My intuition says no, but we will see.

Not trying to start any fights here just looking for what are realistic expectations of a new barrel and if it is normal to have super shitty accuracy for break in then suddenly it behaves well?  

As well as again, with the price of ammo nowadays, how much $ do I need to burn until it is decided it is a bad barrel?

Glad you're bringing a rifle you shoot well.  A known quantity that'll let you know if you're "off" that day.

Rainier barrels are generally a safe bet, but everybody produces a lemon from time to time.  We need to remove variables to determine if you've got a lemon or not.  

Early on, cleaning a precision barrel is about removing the artifacts that might be present from the manufacturing process.  Those minute irregularities create uneven deposits in the bore.  75 rounds should've smoothed things out some, but a brush plus patches/jags will ensure you're doing everything right on your end.  Once a barrel is broken in, I only clean precision barrels when accuracy falls off.  

Lastly, as others have mentioned, using 2 types of  (fair at best) ammo to gauge accuracy may not give you a complete picture.  Buy some good match-quality ammo and do some more shooting.

The question has been asked, but I’m not sure if there was a reply. Is this your first large frame AR?

Please post groups from your session today if possible.

Results look good so I will go eat crow now.

I was able to find some Remington Premier 168gr match ammo with Sierra Matchkings in a local store so with those I was able to get the kind of groups I have wanted to see.  Best group of 1.03 MOA and two others at 1.86 MOA and 2.13 MOA.  This is shooting off a bipod on the bench with a rear bag at 87 yards.  

In the images below you can see that in all three shot groups I basically have 4 shots touching than one off by an inch or so.



Cant get the link to work so here is an imgur link
Targets

*note that the first target has 6 shots on it since the first 4 were touching and shot 5 dropped a bit (I could tell I slightly pulled it) so I figured I would try a 6th to see if I could do better (but I pulled that one too).  In the end the first 5 were still my best group of the day.


With these results I can answer my question of if the barrel is a lemon or not, it is not, and I will be keeping it.  I am just surprised to see non-match ammo perform so poorly.  I am used to the 5.56 side of the house where match ammo is sub-MOA but regular factory ammo has always been 2-2.5 MOA for me.  

To answer some of the remaining questions:
--This is my first large frame AR

Build details:
--Centurion Arms Mk11 upper/lower receiver set
--Centurion Arms 13" rail
--Rainier Arms 18" Rifle length gas barrel
--Superlative Arms Select adjustable gas block (sticks out due to short rail length and allows me to adjust lever between suppressed and unsuppressed)
--CMC Enhanced BCG
--Geissele SSA-E
--AR10 spring with Armalite AR10 buffer (basically a carbine H3) in a VLTOR A5 tube
--Vltor Emod stock

--Crimson Trace 3-18x optic
--ADM mount

Good to hear!

And FWIW, folks asking if it's your first large frame AR weren't being condescending.  Shooting one of these for accuracy is a completely different animal than anything else.  To be consistent, everything about your form has to be just right.

Good luck shooting your new barrel.  You'll find several factory loads that'll shoot well, so stock up on them when you do.

Results look promising!

I’m not overly familiar with that ammo, so I’m not certain of it’s consistency. Your results with the fliers high/low may be the ammo or perhaps a result of your form. Regardless, the results weren’t bad at all.

With a large frame, if I’m not shouldering the rifle firmly, there’s a better chance that I’ll get a high round flier. This can also be from a magazine/carrier tilt, but I usually assume it was something I did.

When using a bipod, if I’ve had a round go high from not shouldering firmly enough, sometimes I’ll muscle it and load the bipod to ensure I’m loading the rifle into my shoulder and I’ll get a round to impact a bit lower in the group. This also seems to happen when 4 shots are going into the same hole and I’m trying to force the 5th to join his buddies. Consistency is key.

Just some things to consider. Looks like you’ve got a great start!

Personally with the vertical stringing I would try a different target with less clutter and turn my magnification down just a bit on my optic. At higher magnification you might be getting some mirage off the suppressor. I have also done this with people take a plain white piece of paper. Shoot the first shot then aim at the bullet hole. The single aiming point really shows mirage.

OP:  Looks like you are making good progress.  I would strip that barrel bore and throat of copper with a good copper solvent like Barnes CR10 or Sweets, fire a few fouling shots and shoot groups again.  Don't be afraid to use a bronze brush and follow up with good soaked patches until they are no longer showing green copper stain.  Then a dry patch.  Of course, use a one piece coated rod and bore guide.  You probably already know this stuff.

Also, check carefully to make sure your free float rail is not in contact with your gas block or anything else.  Finally, try shooting groups without the can.  POI will likely change, but this will eliminate any variable such as slight contact with the bullet, or barrel harmonic changes that might be affecting accuracy.  When using a bipod, strive for the same preload on every shot.  I do better with a front rest.

All groups shown above were without a can.  And the “best” group was with me having adjusted the gas to be off so it was essentially a single shot with no moving parts.

I was very stable but even with that there was a slight movement equal to about an inch if measured on the target, which is coincidentally about the size of my shot group.

As for the verticality,  the times when I felt that I “pulled” it a little or at least broke the shot not quite perfectly, it seemed to go up or done never side to side.  So that may be part of it.

OK, just for the humor factor ...

try shooting off of a front bag instead of a bipod.  Get the rifle to rest solidly in the bags so you can get consistent head position (no fore-and-aft movement), and break the trigger and follow-through consistently.  

Your groups are taller than they are fat or wide, which means either your velocity is inconsistent; your head and eye distance are not the same, shot-to-shot; your reaction to the rifle recoiling changes; or the forward-loading on your bipod varies enough to give you vertical dispersion.

I’m glad you’re shooting better now.  It can be a big deal when you suddenly don’t get the accuracy you should expect from a fairly familiar platform.

I was going to point out that your can was potentially messing you up.  Not that you want to not use it, but get the hang of the large AR first, then look at using your suppressor.

I was going to suggest that your can could be causing some of your issues, but it looks like others have brought it up.

Another thing to look at is what is the twist rate of your barrel? It looks like it likes the lighter weight bullets better than the heavier ones.

As one who has shot over 20k rounds of M118LR, I can say there are some lots that didn’t shoot very good while others were amazing. We mostly shot 10 shot groups which most often were MOA or better. However, M118LR wasn’t designed to be match grade but to be sub-minute of bad guy and to replace the M118SB. The Special Ball was nothing special about it.