Post Mortem AR Help- Cheap Barrel or Poor Bullet Concentricity?

[Mountain]

My recent AR carbine build will shoot an ok group sometimes, then will spray a round or two darn near off the paper. I took extra care to ensure tight barrel to upper fit yet the random shots continue. I am feeding it somewhat cheap US made 5.56- 55gr FMJ's by Indpendence or same by Winchester, all I could find during the period of full 'tard.

I mounted a decent Nikon scope for checking out the AR. Scope is ok- I have consistently shot 0.3" groups with it on my Windham VEX. Ammo was factory match or benchrest quality reloads.

Barrel is one of the cheap melonite Mossbergs, which I thought might be the culprit. A friend I was shooting with suggested we check the ammo and that I shouldn't be so quick to judge the barrel. He had a concentricity guage and started checking. Independence concentricity varied from .002" to .008". Surprisingly the Winchester rounds ran as high as .010" out, with most at the higher end of the range. My bud thought these might be tumbling by 100 yds, and probably would be headed for the sidelines by 50 yds.

What say the forum- crap ammo, crap barrel, or both? Thanks...

 

[Supermoto]

Why don't you put some of your .3" ammo through it and see how it shoots

 

[noddaduma]

I say you need to learn to reload and work up an accurate load.

 

[Fattman]

Twist on your barrel? 1/7 likes a little heavier ammo

 

[Mr. Brownstone]

Does it happen every other round? I was taking a class at Sig once. A guy had brought a brand new stag. Every other round was missing the steel plate at 100. Turns out, one of the feed ramps had a small machining tool mark, or a burr on it. So every other round was scraping on the way in and scratching the bullet, sending it off course.

 

[Mountain]

Thanks for the responses.

Yes, plan to shoot some match ammo later today or perhaps tomorrow.

Regarding reloads- I have been reloading the benchrest quality rounds but not the plinking rounds. More or less I have been burning up cheap factory stuff in the carbine.

Good advice to check the feed ramp.

 

[bfm]

Twist on your barrel? 1/7 likes a little heavier ammo

It really does.

 

[Mountain]

It really does.

Ok, thanks. I'll reload some 69gr and 77gr rounds tonight, and back to the range tomorrow.

 

[bfm]

Ok, thanks. I'll reload some 69gr and 77gr rounds tonight, and back to the range tomorrow.

If the barrel is 1 in 7, you should see better groups when you get up or past 70 gr. If not, there is probably another problem, I would start with checking the barrel crown and the chamber next. Small irregularities in crown can cause a lot of spread.

Even the 62 gr green tips do better at 1 in 9 than 1 in 7.

 

[P-14]

Also, make sure the optic is tightly attached to the rail. While sighting in one of my ARs, I could not get a reliable group. Turns out the optic loosened. Once tightened, problem solved.

 

[stixnstones]

Can you shoot the Windham side by side with the home-built on the same day with the same ammo to compare groups? What about switching barrels or uppers to compare?

 

[Smash05]

I highly doubt it's the ammo, what size groups at 100

 

[Mountain]

Thanks everyone-

Here's a 50 yd target from this afternoon, two groups of 3:

View attachment 109349

25 yard groups were one ragged hole. Difference? Quality loads with 69gr SMK's. I did check the feed ramp area- looked good and mated with the upper perfectly. I'm going to give a to the cheap Mossberg OEM 16" melonite barrels. Here's the AR with my $149 Mosin carbine:

View attachment 109350

The AR was my first build, primarily from NES-found parts (GB, classified, or just a head's up). I'm quite happy with the performance now & would not expect any better accuracy at 50 yds.

Avoid the Winchester 55gr white box 5.56- that's the ammo that sucked the most. It's labeled 'Target', but I think that's a misprint- was supposed to be labeled, 'Miss the Target'.

 

[Terminator03]

Interesting on the ammo. Thanks for the update.

 

[Mountain]

Interesting on the ammo. Thanks for the update.

Initially I had thought it was something I screwed up, or a buyer beware lesson from a $89 barrel. Similar poor results from two different sources of ammo seemed to support this.

Today I started at 25 yards because I wanted to be sure all shots were on paper- the prior groups were that bad at 50 yards! First shot on paper, close enough to center- great! Second shot, damn!- where is it? OMFG, can't even hit paper at 25 yards. Oh well, maybe 3rd will be on paper. Damn again, or not... Upon a closer look, all 3 of the shots were basically on top of each other.50 yard shots confirmed problem solved. At this point I'd need to do some trigger work to further tighten the groups. The current pull is a little stout. Might try some 77gr bullets later as well.

 

[dcmdon]

Why don't you put some of your .3" ammo through it and see how it shoots

Exactly.

Then try some of the Independence ammo through the .3" gun.

***
Edit. oops, posted before I read through to the end. Congratulations.

By the way. Upper to lower fit has pretty much nothing to do with accuracy.

Don

 

[bfm]

This is one good reason to have multiple ARs, or at least uppers. 55grain ball was originally used with a 1 in 14" twist. Marines insisted on a 1 in 12 only because of accuracy when it was below freezing. I have slower twist ARs so I can shoot cheap Federal 193 all day and higher rate for longer range or match shooting with heavier BTHP rounds.

 

[dcmdon]

BFM
Thats a great point. And to take it further, the current M855 62 gr stuff usually shoots better out of a 1 in 9 twist barrel than a 1 in 7, which the Internets says is superior.

In reality, the 1 in 7 twist doesn't really shine until you approach 70 gr.

 

[bfm]

BFM
Thats a great point. And to take it further, the current M855 62 gr stuff usually shoots better out of a 1 in 9 twist barrel than a 1 in 7, which the Internets says is superior.

In reality, the 1 in 7 twist doesn't really shine until you approach 70 gr.

Post 9,

 

[FreeWillie]

BFM
Thats a great point. And to take it further, the current M855 62 gr stuff usually shoots better out of a 1 in 9 twist barrel than a 1 in 7, which the Internets says is superior.

In reality, the 1 in 7 twist doesn't really shine until you approach 70 gr.

Finding the "sweet spot" is always a little trial and error. I have a NEF Ultra Varment in 243 with a 1:10 bull barrel. I defy you to hit a boxcar with it at 100 with a 55 gr load. At 90 grs you can shoot the antenna off a fly. Totally different rifle with a heavier bullet.

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