Vertical Stringing on an AR

[BigMike]

What would cause a very consistent but 10 inch vertical stringing on an AR at 25 yards? The rifling on the barrel looked almost gone. The gun belongs to a buddy and I figured there was a reason he asked me to zero it. He got the same results and we are both shooters and we swapped optics. Time for a barrel?

 

[jwc0305]

I would say so.

 

[Jambo]

We need more information on the rifle. Free float tube? Plastic handguards? Non shooter induced vertical stringing can be caused by something on the top of the barrel "binding".

For example, the front sight post could be mounted in such a way that the top handguard is bound in place. This would artificially push the barrel downward. When the barrel heats up the barrel gets longer, relieves the pressure and the downward push, and the bullets begin to impact in a vertical string upward, until they reach the natural "non binded" point of impact. Basically, once it gets hot it should shoot a good group, just not where it did before.

So check it with no handguards, if it has handguards. If it's a free float tube look for something pressing against the barrel. I've seen a slightly long screw for an MLOK component press against a barrel and move it a LOT.

I wouldn't expect vertical stringing from a worn out barrel. I would expect a larger "pattern" instead.

 

[hdgapeach]

^^^ What Jambo said. You can verify the heat / barrel movement thing if you have some time to kill: Shoot one round at target. Let barrel completely cool down, then fire a second round. Reapeat the complete cool down and fire a third. If results are an acceptable three round group (with no verticle stringing), you'll know you have a heat issue with an external influence on the barrel.

Hey, all you got to lose is time and three rounds (lol).

 

[RobsG]

Heat. That’s been my experience. Of course a hot barrel can expand and contract in a few areas. So it’s about impossible to say 100% for sure exactly where the heat is affecting accuracy. Here is a 10.5” 6.8 BCA barrel I had cut down from a 16” M4 profile. It’s a bit on the skinny side. About 50-75 rounds into the session it showed stringing shots. This is a 100 yard target...

 

[RobsG]

And at 300..

 

[BigMike]

We need more information on the rifle. Free float tube? Plastic handguards? Non shooter induced vertical stringing can be caused by something on the top of the barrel "binding".

For example, the front sight post could be mounted in such a way that the top handguard is bound in place. This would artificially push the barrel downward. When the barrel heats up the barrel gets longer, relieves the pressure and the downward push, and the bullets begin to impact in a vertical string upward, until they reach the natural "non binded" point of impact. Basically, once it gets hot it should shoot a good group, just not where it did before.

So check it with no handguards, if it has handguards. If it's a free float tube look for something pressing against the barrel. I've seen a slightly long screw for an MLOK component press against a barrel and move it a LOT.

I wouldn't expect vertical stringing from a worn out barrel. I would expect a larger "pattern" instead.

Removed the handguard and experienced a 6 inch grouping at 15 yards from a cold barrel. The barrel is a FN govt profile that I found out today came off a machine gun.....

 

[Tracer847]

You said the rifling appears to be almost gone....well that’s kind of a big deal right there. Riffling wears more toward the chamber end...starts as throat erosion and continues onward. All the talk about free floating is pointless if your observations are accurate

 

[ReservoirDawg10]

Heat isn't going to cause vertical stringing like the OP is getting. Not at 25 yards or anywhere close to it. Groups open up with heat, not sting vertically.

 

[GeauxLSU]

Vertical stringing is normally caused by ammo. That is to say inconsistent velocities.