Ar10 double grouping

[comanche91]

Has anyone e experiences their AR10 style rifle grouping two separate groups while maintaing the same point of aim? I have one I built that will shoot .8 as long as I single feed but if I load 10 rounds in a mag and fire it prints two distinct groups. One group (rounds that feed from the left of the mag) will have a POI center and the other group (feed from right side of the mag) will be 1-2" right of my POA. I have tried several mags and it still does it.

any recomendations?

 

[Strikerkc]

Sounds like maybe the head spacing is a little too small; allowing for rounds chambered one at a time or pulled from a specific side to sit straight, but the other side to be wedged in and cocked enough to uniformly throw off aim, but not tight enough to prevent the rifle from cycling?

Maybe a bur or deformity in the chamber causing the same issue?

I assume the shots are at 100 yards?

 

[Lowlight]

That is usually the shooter moving ... you don't realize it you are doing it, but you are moving, aiming at the same point is not the same as making small adjustments in your position.

When you see a group that has 2 round touching and then maybe 3 rounds touching a little bit away, that is the shooter

 

[comanche91]

Had 3 shooters with the same result, its not a little bit away it about 2" to the right of the POA/POI. When you single feed the round all the way in the chamber and let the bolt go home it will print .8 groups. I will continue trouble shooting it and if I can find the root cause I will report back.

 

[bigjake83]

You're canting the rifle, get a vortex anti can't device and you'll be good to go.

 

[padom]

I would say shooter induced as well. Single feeding is not the same animal as driving a large frame AR that's stripping rounds from the mag.

 

[comanche91]

I will say it's not shooter induced. I can grab another large frame AR and stack rounds built in the same configuration . It's isolated to this singular rifle. I have built several large frame ARs and this is the only time I have seen this.

 

[Mike_in_FL]

I will say it's not shooter induced. I can grab another large frame AR and stack rounds built in the same configuration . It's isolated to this singular rifle. I have built several large frame ARs and this is the only time I have seen this.

Have you tried simply chambering from the magazine and ejecting the cartridges unfired to check for damage? It would be interesting to see if the feed ramp on the right is damaging those cartridges somehow. I've had bullet jackets damaged by bad ramps but maybe it's simply a knocking them unconcentric.

 

[Strikerkc]

So. If the only time it shoots to the right, is feeding the round from a certain angle in the mag. Which means there's got to be some sort of issue with the barrel/head space area.

You say you built this rifle? You could try breaking the upper back down into parts and reassembling. The barrel might be at a slight angle some how, that lines up with the different sides of the mag differently. If that doesn't work, try any other spare barrel you have? See if things change. Not helpful advice if you didn't build it personally, or have spare barrels sitting around, but it's all I can think of without some serious measuring equipment at your disposal. =/

 

[Strikerkc]

Have you tried simply chambering from the magazine and ejecting the cartridges unfired to check for damage? It would be interesting to see if the feed ramp on the right is damaging those cartridges somehow. I've had bullet jackets damaged by bad ramps but maybe it's simply a knocking them unconcentric.

Do this before you rip the gun apart like I suggested ;P

 

[Yondering]

Have you tried simply chambering from the magazine and ejecting the cartridges unfired to check for damage? It would be interesting to see if the feed ramp on the right is damaging those cartridges somehow. I've had bullet jackets damaged by bad ramps but maybe it's simply a knocking them unconcentric.

If it's not the shooter, this would be my guess too, although damage to the round should cause generally poor accuracy, not a specifically different POI.
I seriously doubt it's a headspace issue.

Post a picture of the target so we can see what you're seeing.

 

[jrbet83]

When you check that round for damage, measure the oal too. Doubt it would make that big of a difference, but recoil in magazine and clambering from magazine can change seating depth if there's not enough neck tension.

 

[Rauch Precision]

New to the site, but not new to these types of problems, I would say that you have a problem with the rounds hitting the feed ramp and messing with the concentricity of the bullet. We had a whole discussion on this at a sniper seminar. The only other thing I could think of that would be shooter induced is parallax, though if you are printing sub minute single loading I would say that it narrows it down to the feed ramp. Are these hand loads or factory ammo?

 

[bigjake83]

Sounds like maybe the head spacing is a little too small; allowing for rounds chambered one at a time or pulled from a specific side to sit straight, but the other side to be wedged in and cocked enough to uniformly throw off aim, but not tight enough to prevent the rifle from cycling?

Maybe a bur or deformity in the chamber causing the same issue?

I assume the shots are at 100 yards?

^^^^^^^ this is some of the worst fucking advice I have seen in a long time.^^^^^^

Must be a AR15.COM Transfer..

 

[Yondering]

Sounds like someone doesn't know what "head spacing" is, but is eager to give advice about it.

To the OP - is your gas block close to touching the rail, enough that it might make contact either from barrel vibration or preloading a bipod? That'll sometimes cause a double grouping. Make sure there is plenty of clearance all around the gas block.

 

[mark5pt56]

I've had this with any gun, I shoot, even a bolt, it's just me, not having solid fundamentals. I don't do it all of the time of course.

 

[Strikerkc]

Sounds like someone doesn't know what "head spacing" is, but is eager to give advice about it.

I'm trying to help, and I am apparently using some terminology wrong, and I'm sorry for confusion that's causing.

What I'm trying to say is "the problem has to be between the bolt and muzzle break".

I'm watching this thread, and I keep seeing people say some version of "the op is shooting badly". But the guy has had two other people test the rifle out and gotten the same results. I just don't see 3 different shooters, all shoot rounds from a specific side of the magazine, off target by the same exact distance, in the same direction, due to fundamentals issues. The odds seem astronomical.

​

 

[lennyo3034]

How tight is the barrel extension to receiver fit? How much torque did you put on the barrel nut?

 

[MWM-5150]

If you're confident that its not shooter induced...I would look at your barrel extension and double check your barrel nut. I started with a fresh barrel from Rainier on my 308. It had really sharp edges on the locking lugs of the barrel extension, and it was beating the shit out of my brass and the bullet while chambering the round. Fucked up the groups something awful until it wore in a bit and I did some "Billy-Bob Gunsmith" polishing on the lugs. This not only helped with cycling, but its now much kinder to my brass.

Since your problem seems to be isolated to feeding from one side of the mag or the other, I would verify that the lugs are indexed correctly (not canted), and that they are of consistent in shape and finish. Look for burrs or sharp edges that could be damaging the bullets during feeding.

Also, wouldn't hurt to double check your optic setup. Make sure your rings are all torqued correctly and that your optic is maintaining zero under recoil.

 

[Greg Langelius *]

Not seen any mention yet about the ammunition source.

Are we talking about handloads?

If we are, I'd suggest that the charge and the barrel's harmonic node could be out of sync. This is one source of double grouping. If the load charge weight is altered up or down by a mild degree, does the outcome change with it? This can still exist with factory ammo, but well crafted handloads will generally have a smaller size for the two individual groups due to more precisely equalized charges. If factory ammo, try a different one, any different one. If it's ammo, the double grouping ought to change.

Another would be bedding (in a bolt gun). But before we dismiss bedding, let's stop to consider how the effect of a loose relationship between the Upper, Lower, and Pivot/Disassembly Pins just might mimic the effect of loose bedding. Don't forget to check for sideways slop, too; it could well be linked to this posted issue. This also relates to any motion in the optics mount and/or a loose reticle. The relationships between rifle groups under recoil need to be consistent. Slop translates into different barrel attitudes at the time of bullet emergence. It doesn't take much when one considers that the difference between a 100yd zero and a 1000yd zero is roughly 30 minutes, or 1/2 of a degree. If any slop between Upper and lower can be detected, there's a problem, and may be one of the key reasons why otherwise precisely built AR's may not shoot as tight as logic might suggest.

Understand that the aim of the rifle at the time of bullet emergence is always different from what exists when the sear breaks. The physics of recoil under third law principles insists that this is so. That is why component fit/integrity, consistent shooting position and grip, and other factors become so critical once we have already nailed down the mechanics of load development and production, and rifle configuration. When we can't identify a problem, are we also taking a moment for looking in the mirror? By definition, any problems humans can have are human problems, in some way or another.

Let's also consider Parallax, if the cheek weld is not clearly defined and the parallax adjustment is off because the parallax adjustment and objective focus are combined at the factory, but not in good sync for that particular scope (this is a lot more common than many might think, and is one of the reasons why expensive scopes cost so much more). This might be improved upon by verifying parallax adjustment with the head nod/wag test. If the reticle tracks across the target image in close conjunction with head repositioning, parallax is off. It may be necessary to choose between a sharp focus and a null parallax.

Does it occur consistently with different magazines?

I'm still backing LowLIght regarding shooter input; just 'cause several folks do it similarly does not always mean it's not a people issue. Do they all have a good LOP match? A bad LOP match will very likely generate problems on the target.

Greg

 

[Darklefthand]

Just out of curiosity, how would shooter error just happen to perfectly cause alternate group placement every other round? Those of you suggesting shooter error are saying that he's posturing a particular way when it's a round loaded from the left side of the magazine, and then differently when the round comes from the right side of the mag... And that his shooting form is being CONSISTENTLY INCONSISTENT, throughout entire magazines? As well as other shooters following the same alternating pattern? I find that mathematically improbable, if not altogether impossible, if we are to trust that this is happening -exactly- as the shooter states.

I suppose one way to test this theory would be to chamber a round from the left side, shoot, then clear the round loaded from the right side of the mag stack. Then shoot the round loaded from the left side of the magazine again, and follow this process, to see what kind of grouping it produces. As shooter states, when he single loads rounds without the magazine, his groups are tight.

My vote is to check the OAL of bullets loaded from alternating sides of the mag, and also look for any sort of damage to the bullet itself, cause by improperly machined feed ramps.

 

[redneckbmxer24]

Just out of curiosity, how would shooter error just happen to perfectly cause alternate group placement every other round? Those of you suggesting shooter error are saying that he's posturing a particular way when it's a round loaded from the left side of the magazine, and then differently when the round comes from the right side of the mag... And that his shooting form is being CONSISTENTLY INCONSISTENT, throughout entire magazines? As well as other shooters following the same alternating pattern? I find that mathematically improbable, if not altogether impossible, if we are to trust that this is happening -exactly- as the shooter states.

Consistently inconsistent is actually an accurate description. As crazy as it sounds this is actually quite common.

I suppose one way to test this theory would be to chamber a round from the left side, shoot, then clear the round loaded from the right side of the mag stack. Then shoot the round loaded from the left side of the magazine again, and follow this process, to see what kind of grouping it produces. As shooter states, when he single loads rounds without the magazine, his groups are tight.

This is actually a good idea. Let the rounds that you think are flying chamber normally (not manually) by firing the round positioned before. Eject that round without firing it until you have 10 or more and then fire at least two 5 shot groups with just those rounds by single feeding those and see what the results are. That will tell you real quick if something from that side is jacking the round up feeding it or if it's just the shooter doing the wrong thing at the right time, every time.

 

[MSTN]

I encountered something similar about 15 years ago, with the rifle printing distinct half-MOA groups consistently an inch apart. Even numbered shots in one group, and odd numbered shots in the other!

The cause was the ARMS #22 throw lever scope mounts slipping on the rail. I suppose any scope mount on a Mil-Std 1913 rail that was insufficiently tight could cause this, as well.

 

[Yondering]

I encountered something similar about 15 years ago, with the rifle printing distinct half-MOA groups consistently an inch apart. Even numbered shots in one group, and odd numbered shots in the other!

The cause was the ARMS #22 throw lever scope mounts slipping on the rail. I suppose any scope mount on a Mil-Std 1913 rail that was insufficiently tight could cause this, as well.

This. Eliminate the obvious problems first - something's loose, or the barrel or gas block touching something, before jumping to unlikely conclusions like several people being consistently inconsistent the same way. That's just basic problem solving; a skill that seems to be sadly lacking these days.

 

[WarbirdWeaponry]

If your POI changes based on which side of the mag has a loaded round the problem is the front pivot pin. None of the other issues would provide a consistent POI change. When the hammer strikes the firing pin and ultimately the BCG the upper floats in one direction or the other based on pressure from the magazine.

You could try a different hold technique, create down pressure to counteract this effect but it won't go away until you resolve the pin fit.