Clamp on rail affecting accuracy?

[Cameramonkey]

Its been a while, but I started thinking about it again so I figured I would see what INGO's experts thought.

I have a Bushmaster XM-15. Nothin fancy, just a run of the mill AR. I added a clamp on 3 rail block to the barrel. No big deal until... I took it to an appleseed.

Long story short my shots were all over the place. Just as I would get my sights adjusted, my rounds would take off in another direction. I never did get it under control, and even spent several sessions doing my own thing to work on my zero instead of running the AQT string. The second day it was the same; just as I would get it zeroed, as the gun heated up, my zero would change significantly and the more I shot the worse the POI would change. (and it seemed to get slightly better after a break such as story time, but would go back to awful after shooting a bit)

Here is my question: would clamping something onto the barrel cause accuracy issues? My thought is the clamped item would absorb heat and cause the barrel to heat irregularly and therefore warp a bit. or am I smoking crack?

Not the exact item, but here is what I am talking about.

Tri-Rail Front Sight Barrel Mount Black Leapers UTG Clamps To Three Barrel Sizes

 

[churchmouse]

I am no metallurgist but that should not cause much of an accuracy issue especially if nothing is hanging on it.

What is a the round count on the barrel....est. if need be....???

Ammo used...???

Iron sights...???

 

[Cameramonkey]

iron sights, less than a thousand rounds. (it was used but Bradis had to tell me it was used it was THAT new looking) I was shooting Wolf Brass ammo. (cheap, but not THAT cheap).

What struck me as odd, was the first sets were reasonably dead on. The more I shot, the worse it got. If there was a pause and the rifle cooled down, the POI seemed to get better. But once it heated up again it got ugly.

 

[Bennettjh]

I don't see why it would but what does the accuracy look like when it's uninstalled?

 

[churchmouse]

iron sights, less than a thousand rounds. (it was used but Bradis had to tell me it was used it was THAT new looking) I was shooting Wolf Brass ammo. (cheap, but not THAT cheap).

What struck me as odd, was the first sets were reasonably dead on. The more I shot, the worse it got. If there was a pause and the rifle cooled down, the POI seemed to get better. But once it heated up again it got ugly.

Again, no expert but that is not good. Have you taken that piece of and ran the gun.......that is the next step. If it does not change I would take a serious look at the barrel, rifling etc.

Also, try some better ammo. It might not like what you were running in it.

The cooling off getting better is not a good sign.

 

[Hop]

Well it's mass so it does effect harmonics of the barrel. That + something else like: ammo, barrel nut torque, etc...

 

[Cameramonkey]

I don't see why it would but what does the accuracy look like when it's uninstalled?

No idea. Havent removed it. It was intended as a HD gun, but figured for fun I'd try to step up my AS game and try to get my Rifleman patch with a centerfire since I already got one with my rimfire.

Again, no expert but that is not good. Have you taken that piece of and ran the gun.......that is the next step. If it does not change I would take a serious look at the barrel, rifling etc.

Also, try some better ammo. It might not like what you were running in it.

I'll remove the piece and see if I can replicate it on my next trip to Proteq. If it still gets wonky I'll break out the PMC ammo and see if that helps.

Pushing me farther into building a 20" freefloat flattop for accuracy and leave the carbine upper for SHTF/HD use.

 

[churchmouse]

Monkey, if you want to check the barrel nut bring it over.

 

[actaeon277]

Well it's mass so it does effect harmonics of the barrel. That + something else like: ammo, barrel nut torque, etc...

This.
It may not explain all of it, but if you mess with the barrel, you mess with the long range accuracy.

 

[cwillour]

Since you were at an Appleseed, were you slung up & where did the sling attach?

 

[Cameramonkey]

Since you were at an Appleseed, were you slung up & where did the sling attach?

Factory lug just forward of the hand guard. I think I see where you are going with this. I thought about sling pressure as well, but my accuracy was for the most part elevation. I even tried ditching the sling during shooting periods to test it and it didnt seem to make a difference.

I'm also not talking about flyers... the groups were decent, but they would walk as a group.

I'll try to replicate it next range trip without the extra goodies attached.

 

[cwillour]

Factory lug just forward of the hand guard. I think I see where you are going with this. I thought about sling pressure as well, but my accuracy was for the most part elevation. I even tried ditching the sling during shooting periods to test it and it didnt seem to make a difference.

I'm also not talking about flyers... the groups were decent, but they would walk as a group.

I'll try to replicate it next range trip without the extra goodies attached.

As I understand it, sling or forearm pressure doesn't affect accuracy as long as it is consistent, but it does shift POI. Since I figure the barrel is not floated, I might try taking everything off (including the handguards) & putting the mag on the ground for the front support. With nothing extra sitting out there to cause a change in forces as it heats up, I would think and POI shift as it heats up would come down to the barrel itself or the barrel/receiver connection.

I might even try putting a co-witnessed optic on it, which might give you a better idea as to whether there is a measurable shift in the FSP location as the barrel heats up.