AR accuracy problem

[Harold Mayo]

OK...just throwing this out.

I have a LMT MRP CQB with 16" barrel, 1 in 7" twist, piston instead of DI. I have mounted a Nightforce 2.5-10x24 on it in a Larue mount. I am trying to zero it, initially at 50 yards. I'm stuck at that range because I can't get any consistent accuracy out of it and can't figure out why. At 50 yards, I can't get better than about 3" groups. I've gone through the following:

1. Bolts on the barrel are tightened to the recommended torque
2. Bolts on Larue mount are all tightened to 15 in-lbs and holding well
3. Mounting levers are tight and there is no play in the mount
4. It's not me because I shoot 1-hole groups with my bolt gun at greater distances than that with more recoil

What have I not tried? Everything is new. The only three things that I can think of are these:

1. Maybe I got a bad Nightforce? Not likely but it could happen. Zero could be wandering.
2. I'm shooting 55gr FMJ. Could the 1 in 7" twist not like it because it's lighter? I didn't think that it would be significant but maybe so?
3. People say that piston guns aren't as accurate. Surely it's not THAT inaccurate? If so, it's a defect and not standard.

I'm going to pick up some 75gr and/or 77gr match stuff tomorrow and see if it could be an ammo issue and I'll stick the scope and mount on a DI gun that I KNOW shoots well at that range with that ammunition and see if it's the scope.

Anything else? Could ammunition be that big of a factor? I've shot AR's for years but have used barrels that were 1 in 9" with 55gr and 62gr ammunition exclusively. Only owned one or two AR's that were 1 in 7" and only shot them a few times before trading or selling them.

Anyone care to comment?

 

[ThePunisher'sArmory]

Try zeroing at 25 yrds with iron sights. The POI should be 2-2.5" low of POA and centered. This should also get you close to zero at 100yrds. See if your groups are still large at that distance. That will rule out your optic problem.

 

[Nugilum]

I've got a BCM 16" 1/7" lightweight Middie. I know that BCM has a very good reputation, but this upper can not handle 55gr. bullets. My accuracy goes over 2" at 25 yards.

Sadly, I can't get my hands on anything heavier here in Augusta.

Let us know if the heavies bullets help.

 

[Zak Smith]

First, try several types of known good ammo.

That is the most likely issue.

 

[janobles14]

its the bullet weight. i dumped a 1 in 7 before i figured this out. my neighbor (who bought the rifle) slung 62's and 70-somethings with all kinds of success. we took it out with my next AR (a 1 in 9 and shot that my preferred 55's as well as anything could) and the old 1 in 7 still threw them like a bent barrel shotgun.

 

[Tim the student]

Another vote for the ammunition used. The ammo is probably too light for that gun (but some guys report 55 grains shooting well with a 1/7 - but it sure looks like yours isn't one of them), and FMJ is not known for its accuracy.

I bet anything heavier will shoot better.

 

[Bartholomew Roberts]

I'm skeptical it is the bullet weight. 55gr shoots just fine out of 1:7 barrels out to about 300yds. Past 300yds, you may notice some differences; but even there few people can shoot well enough to appreciate those differences. Here is a good link to a page from an Army field manual discussing the difference between 55gr fired through a 1:12 barrel and 55gr fired through a 1:7 barrel at 300yds (no difference - same exact group size)

I do agree with Zak that ammunition is likely the issue though. You aren't using any Russian ammo for this by chance are you? I've seen accuracy like that from Wolf and other Russian ammo in the past. A good 55gr ammo to try (at least for accuracy purposes) is the Hornday training ammo.

 

[lencac]

I think the you should start by eliminating as many variables as possible. Shoot the iron sights at 50 with some match grade 69gr. or 75 gr. ammo. At 50 that rifle should be a one holer. Even a guy who is only a decent shot at 50 should be able to get 1 MOA. If it still shoots the same way then you got a problem with the rifle. And I would put my money on it being the crown.
I have a scoped 1/7 Colt H-BAR and it doesn't really shoot 55gr. stuff great but it will still shoot 1.5 MOA off the bench at 100 yrds. On a good day it will shoot 68, 69, 75 gr. .5 MOA off the bench @ 100 yrds. And don't forget your Accuwedge.

 

[Al Thompson]

Harold, ammo is my guess too. I have had some issues with Wally World Federal 100 round packs - depending on lot numbers. I have had much better success with PMC ammo. Three inch groups at 50 yards was exactly what I was seeing.

Nugilum, drive east to Columbia to Palmetto State Armory's retail location to get a better selection of ammo.

 

[Rokman]

I also agree that you need to try some quality ammo of 69-75 grains.

 

[Harold Mayo]

Well...it wasn't the equipment or me, it was the ammo.

Worked up from 62gr FMJ to 75gr BTHP and saw marked tightening of the groups. The 62gr was actually noticeably better than the 55gr FMJ. 69gr BTHP was pretty good but still not grouping like I thought it should. Finally, the Hornady match stuff gave me groups at 50 yards that were the size of my thumbnail. Didn't go more than that as I was just checking to see how much the ammunition mattered. Apparently quite a bit.

I thought I knew a bit more than I actually did. I didn't think that I would see 3" groups at 50 yards like that with the lighter bullets and faster twist. I knew that there would be SOME difference but not like that. I'll try some more ammunition but I'm not sure that this is a keeper. Great gun but sometimes I want to plink with an AR and, at $1/round for the ammo it likes, it's not a plinker.

Thanks to all who replied.

 

[taliv]

harold, i have a lot of ARs, almost all with 1:7 twists. for whatever reason, my fav ammo prvi m193 55g works great in all but one. that one happens to be my favorite AR the KAC SR-15.

When i first bought it, i had exactly the same experience. 3" groups at 50 yrds. i went back and double checked the swaro optic, larue QD, etc just like you did. Then i tried 3 rounds of my 80g SMK handloads that i shoot from the 600 yrd line in my service rifle. They went through the same hole, to my surprise.

I have no explanation for it. The gun shoots other ammo ok. the ammo works in other guns ok. But clearly the combination of my fav gun and my fav ammo does NOT work. So I handload bulk 75g for it and get good accuracy. (about 1moa)

it happens sometimes.

 

[kwelz]

While 55 Grain is not a problem for 1/7 twist barrels, some guns don't like lighter bullets like taliv said.

 

[Rail Driver]

I don't know if it makes a difference or not, but from what I've seen, the chrome lined bores in 1:7 tend to like the heavier ammo, whereas the nitrided barrels eat it all. It may have something to do with the lower friction coefficient of the chrome lined barrel not giving the lighter rounds time to stabilize properly?

 

[rcmodel]

Try better bullets.
FMJ is the least accurate bullet type made by anyone.

A bullet with the jacket hole in the nose will almost always have a very flat uniform base.

A FMJ has the jacket hole in the base, and simply cannot be flat & true, so they go wobbling off down range as they clear the muzzle crown differently shot to shot.

My old Colt carbine shoots 2 1/2" or worse with 55 grain FMJ, and under MOA with 55 grain Hornady V-Max or Nosler Ballistic Tips.

rc

 

[wanderinwalker]

Sounds like you narrrowed down your problem. Just for fun, track down some 52gr match hollowpoints sometime and try them. I would be shocked if they grouped as badly as the 55gr FMJs did. It's not just bullet weight to barrel twist, it's also a lot of bullet QUALITY (severely lacking in bulk FMJs IMO).

 

[TonyAngel]

1:7 twist barrels are not inherently inaccurate with lighter bullets. The problem is that the fast twist exacerbates the wobble that cheap bullets are going to have because they aren't balanced as well as good bullets. M193 is crap ammo for target shooting and even worse coming out of a 1:7 twist barrel. As has been said, try some 52gr match bullets and watch those groups tighten up.

For an inexpensive alternative to M193, try Hornady's 55gr FMJ Practice ammo. The steel cased stuff. It's loaded by Hornady with their projectiles, which are much better than the mil spec crap that M193 is loaded with.

You might also want to look into the trigger. My groups tightened significantly going from a stock trigger to a Geissele SSA and got better when I went to a Geissele SSA-E.

 

[killchain]

That's your ammo, I think.

I have a 1/9 barrel and if I feed it 55gr it shoots like a blunderbuss. Put some 62gr in it though and it's a tack driver.

 

[ms6852]

Like every one says. Try the heavier weighted bullets with a 1-7 twist it may only like 75gr and 80 gr. You may start with a 69 gr to see if it likes it.

 

[kwelz]

MS6852 I don't think that is what anyone is saying.

 

[possum]

First, try several types of known good ammo.

That is the most likely issue.

i agree, and this is where i would start. with a 1 in 7 twist i would use some 75 and 77gr.

 

[Zak Smith]

I didn't say necessarily heavier ammunition, just different. 68-77gr match ammo from Black Hills and Federal are certainly great candidates, though you're almost just as likely to find some lighter ammo that shoots good too. The main point is to try some different types of ammo to find one your rifle "likes."

 

[TonyAngel]

I'm shooting a 1:8 twist. At 100 yards, I get better results with the 69s than I do with the 77s. Get some money together, go to ammunitiontogo and get a selection of Black Hills. I like heavier bullets more than lighter ones. I tried the 68s, 69s, 75s and 77s. My barrel seems to prefer Sierra over Hornady. Well, maybe it was just my trigger finger.

 

[Bartholomew Roberts]

Your rifle may not like that particular type of 55gr ammo; but I doubt the twist is the issue. I'd just try a different brand of cheap 55gr. Give the Hornday Training ammo a try - it has great accuracy and a decent price, though I have seen some rifles have problems with its steel case.

 

[MCMXI]

3. People say that piston guns aren't as accurate. Surely it's not THAT inaccurate? If so, it's a defect and not standard.

Here's a 10-shot group (2.5") shot this past Sunday at 100 yards in around 45 seconds using XM193 (55gr FMJ). This rifle (a gas piston AR) has a 1:8 twist and 2-1/2" groups seem to be the norm at 100 yards with XM193. With Hornady 75gr HPBT bullets and Varget powder, the groups shrink considerably, but for combat accuracy XM193 works quite well. I chronographed 41 rounds this past weekend and the average velocity was 3,169 fps out of my 16" barrel. Federal lists the velocity at 3,240 fps.