Weird Accuracy Issues with AR Build

[BCP]

I have a rifle I put together a little while ago and it's been shooting really weird. It's a bcm upper, green mountain barrel, KAC URX handguard, set screw gas block, microbest bcg, lower is Palmetto with a larue trigger and lmt sopmod. It has weird fliers that I can't account for and it doesn't seem to hold zero. It doesn't shoot to point of aim consistently.

Things I've tried:

Rebarreled
Took it all apart, put it back together, different barrel nut torque (fiddled with upper receivers and found one to thermo fit, didn't help)
Factory ammo to eliminate handloads
Two different scopes, irons
Less oomph on gas block set screws
No muzzle device
Lead sled didn't help

Now here's the weirdest part, I shot it single fed no magazine and it grouped well vertically BUT it had a horizontal stringing of about 2moa

So I'm pretty much stumped, I've built probably 40 ARs and this one seems to be possessed or something

 

[Redlion]

What do you mean by re-barreled? A different barrel or the same one that you put back on with a new/different torque?

 

[BCP]

What do you mean by re-barreled? A different barrel or the same one that you put back on with a new/different torque?

New barrel but it shoots exactly the same way

And then I thermofit it so now it's gonna be hell to take apart lol

 

[RTH1800]

Caliber? Large or small frame?

 

[.505gibbs]

Are you sure your scope mount is good? I had a Bobro that likely cost me a thousand dollars chasing other components and ammo.

 

[Hecouldgoalltheway]

Have you heard of the theory of dissimilar metals?

I think having kac parts and palmetto state parts on the same rifle might be creating an imbalance.

 

[Bob_Johnson1965]

I had a similar problem. Rebuilt the entire gun, new barrel, upper, lower, handguards, stock, everything. Fired over 900 rounds in load development.

The ONE thing I didn't change was the scope itself. Yup, turns out the scope was defective. SWFA 12x42. I never thought a fixed power scope would be defective, but I was wrong!

 

[BCP]

Are you sure your scope mount is good? I had a Bobro that likely cost me a thousand dollars chasing other components and ammo.

Thought that too but it’s been through a couple mounts.

 

[Veracity]

Gas block, mlok screws, gas tube or anything else making contact with rail/barrel?

Green Mountain have been pretty awful barrels in my experience.

 

[BCP]

I've had decent luck with green mountain AR barrels they all shoot 3/4-1" with hornady match, this is almost like it has a bad scope or something is terrible loose yet I can't find anything loose and I've been through two scopes and a set of iron sights.

I had a similar problem. Rebuilt the entire gun, new barrel, upper, lower, handguards, stock, everything. Fired over 900 rounds in load development.

The ONE thing I didn't change was the scope itself. Yup, turns out the scope was defective. SWFA 12x42. I never thought a fixed power scope would be defective, but I was wrong!

Having two bad barrels or two bad scopes in a row would be my luck.

 

[Veracity]

I've had decent luck with green mountain AR barrels they all shoot 3/4-1" with hornady match, this is almost like it has a bad scope or something is terrible loose yet I can't find anything loose and I've been through two scopes and a set of iron sights.



Having two bad barrels or two bad scopes in a row would be my luck.

What twist and bullet?

 

[BCP]

What twist and bullet?

8 twist and anything from 55-75gr bullets

Example, I shoot 3 rounds of hornady 75gr match and get a 3/4" group, then another 3 rounds are two touching and the 3rd is 3" away

10 rounds of american eagle 55gr goes into 1.5" round group, then I shot a separate group and have 4" of vertical stringing

Handloads that were 3 rounds into a tiny group from a lead sled are 3 rounds with horizontal stringing of 2"

Other times it will shoot a good group in one spot and then another good group a couple inches away

The only two things that stand out to me are the upper/lower fit aren't super tight and it's overgassed but I've got other AR15s that wiggle and they shoot straighter than this one

 

[Ernest 5.56]

Try another bolt + firing pin

Try another lower

 

[RTH1800]

Do you have an AR that you can shoot well?
Not being sarcastic but they are not bolt guns and require a different skill set to shoot well.

 

[supercorndogs]

Shooting them well, and 4 moa are two diffrent things. They aren't that hard to shoot if you can shoot.

I would suspect gas block hitting the handguard also. This thread sounds like exactly what I have seen that do. I woukd reiterate everything in the post I liked a few above mine.

 

[BCP]

Do you have an AR that you can shoot well?
Not being sarcastic but they are not bolt guns and require a different skill set to shoot well.

I have a bunch this one just doesn’t fucking work lol.

 

[diggler1833]

I have a rifle I put together a little while ago and it's been shooting really weird. It's a bcm upper, green mountain barrel, KAC URX handguard, set screw gas block, microbest bcg, lower is Palmetto with a larue trigger and lmt sopmod. It has weird fliers that I can't account for and it doesn't seem to hold zero. It doesn't shoot to point of aim consistently.

Things I've tried:

Rebarreled
Took it all apart, put it back together, different barrel nut torque (fiddled with upper receivers and found one to thermo fit, didn't help)
Factory ammo to eliminate handloads
Two different scopes, irons
Less oomph on gas block set screws
No muzzle device
Lead sled didn't help

Now here's the weirdest part, I shot it single fed no magazine and it grouped well vertically BUT it had a horizontal stringing of about 2moa

So I'm pretty much stumped, I've built probably 40 ARs and this one seems to be possessed or something

I had this problem once and had an almost identical amount of effort invested into fixing it. It turned out to be that I could manually flex the barrel enough to contact the gas block with the inside of the handguard. It was flexing under recoil and creating errant shots.

Switched from an .875 to a .750 barrel and my problem disappeared immediately.

Maybe not your issue, but worth a look.

 

[BCP]

I had this problem once and had an almost identical amount of effort invested into fixing it. It turned out to be that I could manually flex the barrel enough to contact the gas block with the inside of the handguard. It was flexing under recoil and creating errant shots.

Switched from an .875 to a .750 barrel and my problem disappeared immediately.

Maybe not your issue, but worth a look.

Naw no wiggle no marks inside handguards from contact either

I'm gonna try a different lower if that doesn't work then it's tear it all apart and start over time

 

[hlee]

When you think you’ve eliminated everything else, hand it to a different shooter…

 

[bfoosh006]

If you take it apart again... I'd suggest lapping the upper receiver face.

Easy to do, and since you are there.

 

[fedupflyer]

Torque values used on the barrel nut, gas block and muzzle device?

 

[BCP]

Torque values used on the barrel nut, gas block and muzzle device?

I fiddled with those when I put it back together the second time still same results. The last thing I have to check is the lower.

 

[hlee]

Unless the rifle wobbles around like a clapped out lawn chair, there’s really nothing in the lower that affects accuracy.

 

[BCP]

Whomever said different bolt/firing pin gets a cigar.

 

[Ernest 5.56]

Whomever said different bolt/firing pin gets a cigar.

Guessing something with firing pin protrusion ?

 

[BCP]

Guessing something with firing pin protrusion ?

I forgot it had a Frankenstein with a wmd bolt and diff carrier. Put in a regular milspec bcg and it shoots straight again. No idea why I’ll have to check headspace gauges protrusion all that stuff.

 

[N_Turs]

Like the post above the lower will have no effect on the upper and accuracy. Like others have suggested Id be willing to bet the gas block is hitting the handguard. When you visually look at it how much clearance is there between them? Try and flex the barrel and handguard with your hand. It will flex they all do. Some of the other suggestions IMO will not fix your problem. Turque specs on barrel nuts, gas blocks, muzzle devices, facing the receiver ect. From what I have seen have very little effect on the accuracy and from what you are describing with the groups wont do anything to improve them.

Your issue is something bigger then a tq spec or faced receiver. While I do agree all of these things being perfect does help squeak out every last bit from the ar platform its not gonna fix whatever you have going on.

All this talk about ar's being harder to shoot then bolt guns is also silly, sure bolt guns are definitely more accurate but Ive never had an issue getting an ar to shoot. If you know how to stay still the guns gonna do what its gonna do... Curious to see how this gets resolved.

 

[gopherslayer]

Like the post above the lower will have no effect on the upper and accuracy. Like others have suggested Id be willing to bet the gas block is hitting the handguard. When you visually look at it how much clearance is there between them? Try and flex the barrel and handguard with your hand. It will flex they all do. Some of the other suggestions IMO will not fix your problem. Turque specs on barrel nuts, gas blocks, muzzle devices, facing the receiver ect. From what I have seen have very little effect on the accuracy and from what you are describing with the groups wont do anything to improve them.

Your issue is something bigger then a tq spec or faced receiver. While I do agree all of these things being perfect does help squeak out every last bit from the ar platform its not gonna fix whatever you have going on.

All this talk about ar's being harder to shoot then bolt guns is also silly, sure bolt guns are definitely more accurate but Ive never had an issue getting an ar to shoot. If you know how to stay still the guns gonna do what its gonna do... Curious to see how this gets resolved.

It was resolved 15 minutes before you posted. It’s the post above yours.

 

[N_Turs]

Fuck me, I don’t know how I missed that post. Glad it was sorted out for you

 

[TonyAngel]

Thermal fitting and/or truing the receiver will likely help you to wring out a bit more accuracy from a good barrel. Neither will turn a 2 MOA shooter into a sub MOA shooter.

I think your problem goes beyond the assembly process. Honestly, I think you should try a better barrel. I've found green mountain barrels very hit and miss. My last experiences with them were more miss than hit.

 

[TimK]

Hmpf. It's odd that the bolt was the problem. If you ever figure out how it was affecting accuracy/POI, let us know.

I had a Valk that I chased for a couple years. Never did figure it out. Rebarreled to 6ARC (everything else the same) and it's perfect.

 

[joe777]

Good to hear it's fixed. Second time I've heard of the bcg being the problem

 

[BCP]

Hmpf. It's odd that the bolt was the problem. If you ever figure out how it was affecting accuracy/POI, let us know.

I had a Valk that I chased for a couple years. Never did figure it out. Rebarreled to 6ARC (everything else the same) and it's perfect.

The bolt I had swapped in was short on headspace so I'm hoping it was just that but yeah rebarreling is on the menu too, probably craddock rtr

 

[TimK]

The bolt I had swapped in was short on headspace so I'm hoping it was just that but yeah rebarreling is on the menu too, probably craddock rtr

My Valk had a Bartlein barrel from Craddock. He was deeply involved in trying to figure it out. Never did. He made me a Bartlien for my 6 ARC, and it's the most accurate AR I've ever owned. Love that guy.