POI Shift with "Hot" Colt Pencil/LW Barrel...

[Dennis]

I just upgraded an old Colt lightweight 14.5" pencil barreled upper with a Troy Alpha rail, Aimpoint T1, MBUS, CTR, MOE+, RVG, and a SFX300 for a fully accessorized carbine that is only 7.5lb when unloaded! (At least by my beat up food scale.)

I went to do initial function testing and sight in at an indoor 25Y range last night and it easily sighted in with consistent <0.5" groups with Black Hills Red Box 75gr ammo. However, after a quick 30rd mag for functionality the POI shifted up and to the left 1" and stayed there for at least a couple minutes. After 10 minutes of cooling the group had shifted about 1/2" back towards original POA. Another quick 30rd mag and the group went back to 1" up and to the left. After cooling for 20 minutes the group was back to original POA. The group size was a consistent <0.5" throughout.

I am sure this is normal, but then that means I will be up to 4" up and to the left when shooting at 100Y, and 8" at 200Y after only one quick mag through the barrel...

I was also shooting a Lilja barrelled upper at the same time and the same test yielded no discernible POI shift at 25Y.

Has anyone else seen similar results? Do you have a pencil/LW barrel that shoots better then this when heated up? DD, BCM, other?

Dennis.

[Sticks]

I am running a Centurion 16" LW Middy, I do not have a POI shift or stringing @ 100y.

I'd do a detailed inspection to make sure you are not getting contact between your rail and barrel or FSB, gas tube. Is there any binding on the gas tube at the barrel nut (side load)

I presume you were shooting off the bench (bipod or rest/bags)? Any of that touching the barrel or FSB?

[djmorris]

I'm surprised your barrel would be getting that hot from a 30 rd mag, even if you fired them all in fairly rapid succession. I'm running an Alpha Rail and have found that it keeps things running a bit cooler on my LW profile than other designs out there.

[M4Fundi]

Shouldn't any POI shift be LOW from a LW hot barrel?

[MistWolf]

Every barrel is a law unto itself. POI shift from heat can be in any direction.

That said, it sounds like something is amiss with your rifle. Either your barrel has internal stresses causing this shift, or something isn't assembled right. The inconsistent return on cooling has me thinking something is loose or is pressing on your barrel someplace

[geohans]

My experience with my Colt LW 16in was that groups opened up, but not in one particular direction.

i.e. going from 2 inches to 5 inches at 100 yds.

[Dennis]

Some answers:

Nothing is contacting the barrel or block (easy to see with the Alpha rail!)
The groups are not opening up, just shifting POI.
It is returning to original POI upon cooling
Checking the gas tube is a good idea, although I just installed it and it was lined up straight (but worth checking again)

I was just wondering what others are seeing with other LW barrels...

Thanks for all the replies!

Dennis.

[!Nvasi0n]

I agree that something is stressing, or urging your barrel to that particular direction. It's not normal to keep a group size and only shift POI. It is normal to open the group up, even after a mag dump of 30...but just a POI shift is odd.

Not that it would be conclusive, but I'd be interested in knowing how your POI shift changes with a different grade of ammo.

Almost sounds like something is inducing a torque on your barrel, like an out of spec upper, or nut? I don't know an won't guess.

Hopefully someone here will get your squared away

[Brahmzy]

While I've never used one (never thought I needed one) I've always been interested in one of THESE (http://www.brownells.com/.aspx/pid=20220/Product/AR-15-M16-UPPER-RECEIVER-LAPPING-TOOL)

I pretty much use MEGA receivers exclusively and have found them to be the absolute best machined receivers I've seen. However, we're talking about microns here... so any receiver could be a culprit. I think I'm going to get one as I build a LOT of rifles.

And maybe a LW barrel is that much more sensitive to something like this (an out of spec upper rec). We're grasping here, obviously, but it might be worth a shot and CAN'T HURT ANYTHING except the wallet.

Regardless, I'd back-off/retorque the barrel nut and take the upper down. 4 MOA shift is SIGNIFICANT. I wouldn't be satistfied until it was fixed.

[advntrjnky]

I have one and it has worked well to square up a few of my uppers (don't have a Mega, though). a number of years ago a barrel smith on another site posted pictures and findings on the squareness of AR15 front receiver rings and found that most aren't. his common practice was to chuck them up an turn them square on a jig he made specifically for the job......IIRC he was able to show accuracy improvements. Another trick that I heard works pretty well is to use semi-permanent Locktite to "bed" the barrel extension to the upper. the Locktite is supposed to fill all of those small imperfections and make a nice rigid unit that won't wander under stress......this accurizing tip has been attributed to Bill Alexander and his work with the 6.5 Grendel.

advntrjnky