AR-15 accuracy issues

[jr45]

For any of AR experts, any advice would be great.

I am a new AR owner. I have a used DPMS M4 w/A2 upper in 5.56 and having some serious grouping issues. Took it to the range today and was firing American Eagle and Tula 55gr ammo (both brands of ammo had the same results). I have fired different ARs prior with good accuracy results so this has me confussed.

From the bench rest, the shots would not group and the impacts would vear all over the place (8-10" up, down, side ways, etc) at 50 yards and double that at 100. When I would adjust the sights, the shots would vear greatly as compared to what it should do (i.e. 1 click on the front post would cause the shots to rise 5+ inches). I do not know the rate of twist as the barrel does not show it (was painted by previous owner). Rifling looks o.k. and the sight bases appear tight and was firing from a rest with the handgaurds touching the rest and I ensured the sight picture stayed the same. Any adivice?

 

[taliv]

i'm trying to figure out what an M4 with A2 upper is but it sounds like you got hosed. if it were new, i'd be sending it back, but that is unlikely now, especially if it was painted by prev owner

try a heavy bullet. get a box of black hills 75 or 77g

 

[jr45]

Like I said, I am new to ARs in general...it has a A2 type upper with 16 inch barrel...I guess that makes it an M4? I do not understand what you mean when you say I got hosed? If the twist rate is designed for a heavier bullet, should it still shoot the lighter versions fine?

 

[Justin]

He's saying that you may have been taken advantage of.

I'm no expert on taking AR's apart or repairing them, but you might check to make sure that the barrel retaining nut is tight. If its loose, it's possible the barrel is moving around when the gun is being fired.

It's also possible the barrel is shot out or just plain bad. Take the gun to a gunsmith to see if they can diagnose the problem.

 

[jr45]

Thanks Justin. After rechecking, the rear sight base does seem loose. The previous owner stated he could not hit very well with it, so for $225, I thought I had nothing to loose. I will bring by the smith for a good look over.

 

[taliv]

right... i think the previous owner screwed you. my guess is that either the sights are loose or the barrel is bad. you said the sights were tight and the rifling looks ok, but unless you have a bore scope and/or air gauge, i doubt you could see defects just staring down the barrel.

another thing you could try is to see if there is some burr in the barrel where the gas port was drilled. try running a piece of cotton through it and see if it snags.

there's more to being an "M4" than just a flat top and 16" (or 14.5") barrel, but I understand what you're saying now. Don't sweat the terminology

i have many 1/7 twist barrels. most shoot 55g just fine. one in particular does not. it shoots roughly 4" groups at 50 yrds with 55g ammo, but will put three 80g bullets through the same hole at 50 yrds. so i just suggested that you try a different weight and see if the groups improve.

 

[taliv]

btw, traditional A2 style rear sights and even the national match style sites (before being pinned) have some play in them. you can rotate them perhaps a degree or so back and forth.

use common sense and see if you think the movement in the sight is enough to result in an 8" move at 50 yrds

 

[jr45]

I can rotate the rear sight a few degrees back and forth but still do not know if this is the source of the problem. The rifling looks good but I do not have a scope to check barrel. Worse case, I could get a new barrel installed.

 

[taliv]

my guess is you'll have to.

but before you do, scrub the crap out of it with something really strong and bore paste or foam
and try a heavy bullet

 

[WNTFW]

jr45,
If you paid only $225 that is not much for a used AR. So you might make out on it in the long run. That is not a bad price for a complete lower.
What I have seen on 4 different AR's the front sight post wiggle in the front sight base. 2 were Colt HBAR's. I would check for something being loose and moving around. Are the sights in good condition? I did see one where the sight post was off center so a click for elevation on the front changed windage. But shot to shot with no sight adjustment was not bad.
Another possibility would be crown damage or bore damage. Like a previous post said make sure the barrel is not loose. Same for the front sight base.
It does sound like something is flopping around on you.
Good luck and don't get frustrated.