AR-15 POI moving as it warms up; loose barrel or ?

[wyrmzr]

I've spent the last few weeks playing with a new scope on my AR. The scope itself seems to function fine, but I'm running into something that I'd been suspicious of on this particular gun; it seems that the POI shifts as the gun warms up.\
Basically, I need to fire a dozen rounds through it before I start getting the accuracy I'm used to at 100 yds.
This rifle is set up with a YHM hand guard, which included a forged aluminum barrel nut. The specific instructions for this setup tell you to tighten the nut up until it's tight, then turn it a little further to line up the next hole for the gas port. This is a bit different from the pre-stressing that is done with so many other barrel nuts, and it makes me wonder whether that's truly enough torque, or whether I've got something else going on inside the gun that would cause it to change POI as it warms up. The barrel is a bull barrel, stainless and fluted.
I'm wondering how much torque I can get away with when torquing on a forged aluminum barrel nut, and want to make sure I'm not overdoing it. Nothing like stripping the threads on the receiver or the nut itself.
I've heard other people say they've had ARs that weren't accurate until they warmed up, and it really is driving me nuts. I want to make sure I'm accurate right out of the box, as I'd like to use this as my coyote rifle.

[country gent]

Im assuming this is a flat top reciver and floating hand gaurd type set up on this rifle. Is the sling on the handgaurd or barrel? Is the gas tube free and lined up with key not under a bind or rubbing anywhere? Has the barrel been stress relieved? How many rounds are thru this rifle? All of these can cause impact shift with warm up. I doubt the barrel nut is the issue as long as it tight enough but normally barrel nut poor fit tension affects groups from the start. The gas tubes not truly free and floating affects barrel harmonics and can cause this as can it bearing on the bolt carrier key un evenly. A sling on the barrel is obvious as it creates diffrent forces directly on barrel if attached to the barrel. A barrel with stress in it will walk shots as it moves due to heat. My NRA match rifle was very accurate from first string to last string of a match and shot way better than I could hold. I would look at sling first. Then gas tube possibly binding and for pressure points against the barrel. Last is the stress in the barrel. If you shot slow are cold groups comparable to warmed up groups and just in a diffrent spot?

[Gtek]

Everything I remember is two torques to 35 ft. lbs. Then 30 to 80 ft. pounds for final. There are several places on web that detail this with pictures and specific shimming details in pictures if required. Torque wrench 90 degrees on wrench for proper induced value, and I also would suggest proper vise collar on upper and correct tools when attempting to avoid torsional twist damage. A #15 drill bit @.180" or the shank of a 3/32" bit @ about .184" for tube alignment check. Projectile vs. twist?

[wyrmzr]

There's no sling on the gun, so that rules out the one possibility. I think it's time to tear it down and check the gas tube, though.
The pre-torquing that often applies to most barrel nuts doesn't really apply to this one; I double checked on YHM's web site, and they do NOT recommend the torquing sequence for their barrel nuts that is typically required for other barrel nuts. I suspect this is because of the different material used in YHM's barrel nuts (forged aluminum instead of steel).

[wyrmzr]

I pulled the AR apart last night, and the one thing I did find is that the gas tube may have been off center. Torquing the barrel nut a few more pounds helped line it up.
I'm going to the range, hopefully this PM, to test out the results.

[country gent]

One trick is find a number drill or pin gage thatt fits close to barrel nut hole or reicever hole and use it to check the nuts alighnment after tightening.

[DCM]

Who made the sts bull barrel? From what grade sts? Was it hammered, buttoned, EDM or cut rifled? Was it stress relieved?

[wyrmzr]

This is the bull barrel:
http://www.brownells.com/rifle-parts/barrel-parts/rifle-barrels/16-1x8-fluted-stainless-steel-barrel-sku100010671-54617-105798.aspx?sku=100010671

[dkf]

How is the fit between the barrel extension and the upper receiver? Is the gas block making contact with the rail anywhere?

[mulespurs]

It could be loose scope mounts. I have a set of mounts the I tightened adequately, but in improper sequence. I was convinced it was barrel or scope, I traded off the scope and on dissasembly found the the scope had some side to side movement in the mounts because of my improper torque sequence. My mistake wasted a lot of ammo.
Now with properly tightened mounts all is good.

[M-Tecs]

Check to ensure everthing is tight. If that works great. If not plot your shots as you test fire until hot. Let it cool and repeat. If the barrel walks the same direction as it heats up it's a barrel stress issue.

[wyrmzr]

I did get out today, and didn't have the rounds walk low and left as usual. However, with the gas tube re-aligned, it looks like I may have to work up a load again, it's as if the gas tube dragging was enough that I could go slightly lower on the powder amount.
More to come as I get sighted back in and a load worked back up; I'm used to siamesed holes at 100 yards, i.e. holes that have 4-5 rounds in one big jagged hole. Frustrating when you're suddenly getting 4 inch groups with a light trigger and a bull barrel.

[DCM]

Check to ensure everthing is tight. If that works great. If not plot your shots as you test fire until hot. Let it cool and repeat. If the barrel walks the same direction as it heats up it's a barrel stress issue.

Agreed.

[madsenshooter]

If you go the NOE forums, you'll find some lube testing that Larry Gibson has done. Some lubes require several fouling shots before settling in and starting to form a group, other lubes are close from the get go. Might not be your problem either, but it's interesting info.

[zubrato]

Depends on your barrel profile, not torque setting especially if it returns to zero after cool down. Lightweight and m4 profile barrels will have wandering poi once they get hot. Lightweight barrels are better for everyday use Ono, and my hammer forged barrel only really changes impact once I start seeing a heat mirage. I really only notice the difference at 400-500 yards when my groups open up and I'm not consistently hitting a 5"by 7" steel plate anymore.
Let her cool down in between strings of fire, and once the sight picture mirage drops off you're back in the game.
If you're looking for longer strings of fire and high accuracy potential then get a bull barrel. Wouldn't recommend it for a patrol rifle though.

Edit sorry just read the last few posts. Sounds like a barrel stress issue. Which mfg? What process; button rifling or hammer forged?

[wyrmzr]

I'm not finding anything on the manufacturing process; it's a Double Star fluted bull barrel, stainless, with a 1-8 twist.
I realigned the gas tube the other day, but in the process, I seem to have managed to put some pressure on my upper receiver. Now the BCG drags about half way down the receiver. It doesn't stop it completely, so I didn't do a LOT of damage to the upper, but I'm wondering at this point whether to keep shooting it, and let the BCG just wear off the extra few thousandths that it's likely needing.
I've already loosened it up noticeably simply by sitting here and working the bolt back and forth while watching TV for less than half an hour.

[DCM]

I am still leaning twords stress in the barrel.
But now I am wondering what jig you are using to support the upper while adjusting the barrel nut also.

[wyrmzr]

The good news is that the rifle still functions fine, although I now need to test two different powder loads in it. It seems I didn't damage the upper like I thought I did, so that's good.
As far as what jig I was using, I wasn't, as I didn't have it available, and got into too much of a hurry. I simply clamped the upper into a bench vise with some wood blocks and a towel to keep from marring it. I know, not the best way to do it, but it worked.
I fired 90 rounds through it today without it wandering like it did before, so the gas tube seems to have been the culprit.
The next step is to determine which it likes best, the 55 gr boat tail with 22 gr of H335, or the same bullet with 22.5 grains of H335 behind it.

[Gunslingerdoc]

The other way to relieve stress is to face the front of the upper receiver to make sure its square. sometimes there a burn the got anodized in. Also chase the threads, with a die on the upper for the same issue. I have found these steps to be required for consistent sub moa AR builds - not on all of them but it seems to remove the odd ball that wont shoot. Also, double star has never been known for making good stuff - not to say they cant but years ago the same folks were known to by lots of parts that were rejects and sell them at shows - don't know if they still do. you may want to lap the barrel as well