Recently built a varmint gun and took it to the range. Was disappointed with accuracy. Would hold about a 1.5" group at 100 yards.  I could live with 1.5" groups, but every several rounds I have a flier in different directions about 2-3" out.  Tried a variety of 55 grain factory .223 ammo, mostly PMC.  The build is all new:  Magpul stock and handgrip, DPMS .223 stainless fluted 20" bull barrel with crown, DPMS upper (lapped for barrel/torqued), Spikes Tactical lower, RRA two stage (polished) trigger, DPMS BCG, and a new, mid-grade Leupold scope (4X12) and rings (lapped/leveled/torqued).  Ran about a dozen rounds, then cleaned it, repeated this 3 times, with cooling in between.  Just plinking.  Then got serious about accuracy.  With fliers, holding 3" groups. I expected better accuracy. I currently have a standard, low-profile gas block with two set screws.  Only one screw lines up with holes in barrel.  Torqued both.  Can't imagine this is stressing the barrel but I am not sure.  Considering a clamp on, adjustable gas block.  Have broken in a number of new ARs but never when I was seeking accuracy.  Does it take a few hundred rounds in an AR before it settles down in terms of accuracy?  Maybe I just need to plink with another 100 rounds or so and then go for group size.  I will measure the chamber and hand load for it for my next trip to the range.  Any opinions on the gas block?  Any suggestions?  Thanks.

Are you trying to test accuracy with 55gr FMJ ?

If you want to shoot tiny groups, your best bet is to handload.
at 100yrds, it's hard to beat a 52gr. sierra match bullet

If you don't handload, try some varmit loads, like vmax,  they will shoot much better than FMJ

55fmj are not built for accuracy

imagine these winchesters spinning towards the target

Sobering photo.

Thanks for the info.  Some more details:  barrel is free floating and torqued at 40ft lbs, DPMS vented tube, 1-8 twist, target crown, Teflon black coated stainless. Trigger is Rock River Arms, two stage, breaks at 4.5 lbs. Ammo was mostly PMC Bronze 55gr FMJ.  Pushing a patch through the barrel is nice and smooth but I can sometimes feel the gas port when I pass it. I don't believe all the hype about special break-in procedures.  I shoot a dozen, clean the barrel, and repeat a few times.  Scope is a new Leupold VX-R (red dot reticle). Shooting at 12 power. I plan to measure run out/free bore and load a quality bullet to fit gun prior to next visit to range.  I know PMC factory ammo is nothing special but I exected better than 3" groups with it.

Do you consistently shoot better groups with any other rifle?  Another AR?

I'm also a fan of using more shots, 10 or even 20 rounds to evaluate overall rifle/ammo combo accuracy. 3-5 shot groups are for comparing my hand-loads.

I find that no matter how big those larger groups are (10-20 shots), there will be large "bug-hole" developing in the paper near the middle of the larger group.  Use that as the reference to adjust zero.  If at least 1/2 my shots are part of that big hole in the middle, I treat that as my nominal group size and move on, develop another load, and see if it does any better with that particular rifle.

my 2c ymmv

I have a few bolt guns under MOA.  A nice M1A that is a consistent MOA.  I have several ARs and a Sig equivalent but none of them set-up for serious accuracy, so no ARs to compare with.  Have another AR under final stage or construction.  It's nearly all the same components as a DPMS mini SASS.  I'll compare my varmint AR set up to SASS in another week or so.

I have a few bolt guns under MOA.  A nice M1A that is a consistent MOA.  I have several ARs and a Sig equivalent but none of them set-up for serious accuracy, so no ARs to compare with.  Have another AR under final stage or construction.  It's nearly all the same components as a DPMS mini SASS.  I'll compare my varmint AR set up to SASS in another week or so.

The above is not that accurate.

Try some match ammunition or reloads

I have a bushmaster that will do 3 inch group with PMC.  It will do inch with reloads

that is nonsense

I've shot very tight groups with 52 and 55gr bullets out of 1:7 twist barrels

55FMJ is not made for match grade accuracy

get some 52gr Sierra or Hornady match bullets and properly handload them, and you will be amazed

I have some 55gr A-MAX on the shelf so I will load them up tonight.  My barrel is 1/8 twist.  I know gas block tuning is mostly for mechanical performance, but does anyone have any experience using an adjustable gas block for accuracy performance?  I think if I turn down the gas to a bit above the minimal level to ensure cycling, it may reduce overall vibration.  Not just what I feel in the recoil, but the vibration in the barrel/upper.  Maybe help with accuracy?  I have the adjustable gas block but didn't put it on for this build.  Was going to put it on a mini-SASS build in the works.  May put it on this varmint build and give it a try.

.223
1:9 For bullets heavier than 63 gr.
1:12 For bullets up to 63 gr.
1:14 For bullets up to 55 gr.
1:15 For bullets up to 55 gr. driven 4,100 fps or more
1:16 For bullets up to 55 gr. driven 4,300 fps or more

http://www.shootandreload.com/2011/06/15/bullet-weight-vs-barrel-twist/

BOOM, right over my head and away

The only 55 grain fmj bullets that don't suck are made by Hornady.

52/53 grains match bullets are usually the most accurate regardless of twist rate. You heard that right, 1/7, 1/8 and 1/9 twist barrels usually shoot 52/53 grain match bullets better than any other weight.

I have several AR's that would be considered "varmint" by most people, they're actually NRA spaceguns. 24.5 grains of Varget under a 69 grain Sierra Match King hammers in them as well as 23.5 grains of VV-N540 under Hornady 75 grain hpbt match bullets. Both consistently shoot under 3/4" groups from a bench at 100 yards. I fired a 1" group at 200 yards on my first outing with an astounded friend as my witness.

55 grain fmj's are junk. I have refused to buy them for over twenty years now. I have shot some that were gifted to me, I never turn down free ammo If, by chance, you find some that shoot well for you buy any many as your budget will allow. This would be an anomaly of gargantuan proportions not likely to be repeated in your lifetime. Otherwise resign yourself to the fact that match bullets make for match accuracy.

excellent post.

+10000

I'm a 3-gunner, I'll buy crappy 55FMJ for 25 yard IPSC speed stages, but not beyond that.

Agreed! I too shoot the light match bullets(@ any range) in 3-gun matches but I do have to load them HOT to get good results. Precision matches I always load the heavies.

You all were right on better bullets and hand loads. Factory FMJs are trash beyond 50 yards.  Shot some Hornady's today and the groups tightened way up.  They were actually 50 gr V-MAXs.  A few fliers but not bad at all. Was more likely me than the gun, as I am still getting acquainted with my Leupold VX-R (not overly impressed with it).    Shot 7-8 groups.  Some were as tight as .7", some as large as 1.5".  But, again, the larger groups were a function of my eye and the scope, not the rifle.  I will be buying some different bullets to load.  Want to try the Hornady 68gr for longer ranges--3-500 yards. May actually go with Swarovski glass instead of the VX-R.  Not really AR scopes but my eyes prefer them and they are my favorite.

For a baseline try getting a box of Federal Gold Medal match ammo in 69gr. to test.