Diagnosing shift in PoI problem

AR/M4 AR Technical Discussion
1

New build, second time out, appeared to have a “wandering zero” problem. I revisited it today (third time), at 50 yards, from a rest, with the following results:

10 shots: 3" group; PoA = PoI. Acceptable, if not great.

Switched to another PoA on the target, 10 shots: PoI shifted 4" to the left, 2" down, and opened up to 5"

Switched to another scope (sighted in for that carbine): roughly the same result, 5 inches to left, 2 down, and 4 inches in diameter.

Equipment: ammo= Federal M193
Barrel: BCM 14.5" with BC 1.5 brake (pin weld)
Receiver: Vltor MUR (I know it’s wasted on a short bbl, but …)
Handguard: Apex Machine midlength
Gas block: BCM (pinned under handguard)

Occasional light breeze would not account for this.

The first scope was a compact ACOG TA44 (1.5x), Larue mount
The second scope was a compact ACOG TA50 (3x), Larue mount

Could it be that warming the barrel/system caused this dramatic shift in PoI? I will have to bring other ammo to use, perhaps Federal 855.

Any suggestions? Anything I’ve left out?

2

Sounds like something isn’t assembled correctly and may be shifting. I doubt barrel heating would cause any where near that much shift in poi.

Can you have someone else shoot the rifle to see if they have the same result?

3

If you have checked your optics mount and verified barrel nut torque…

Check if from a rest with match ammo.
You have to remove the ammunition and human variables as much as possible to really determine if the gun is at fault.

Typos brought to you via Tapatalk and autocorrect.

4

I’m having a similar issue with a LMT 14.5 with a pinned SEI Vortex. I wonder if the pinning process is causing issues as suspected by LMT. M pinning was done by a highly respected shop.

5

I had something similar happen to one of my rifles. Turned out to be the barrel nut was loose.

6

^^^^^^^^What he said.

7

The first thing I did was check the optics mount. It may be shooter error, or ammo.

But, I have the horrible feeling that it might be the barrel nut torque. I did not do it. I don’t want to blame anyone else, because it was done under my close supervision, but I DID NOT ACTUALLY TURN THE WRENCH.

Also, and I do not wish to blame anything or anyone but myself for this, I had never installed an Apex HG before. Perhaps I accidentally backed off the torque when playing with the HG attachment to the barrel nut clamp.

I can jam Glocks. I can do anything.

I will try all of the above, and then tear down the barrel assembly if I have to.

Any other suggestions gladly taken. Thanks to all.

8

Will you send me a jar? Sounds tasty!

As others have said, check assembly of barrel and ensure optic is secure. What tools was the builder using?

9

My tools. Snugged up using an 18" extension. Minimal torque checked with a Craftsman Microtorque set to 30 ft. lbs. Final torque using a Craftsman beam torque wrench that registered 55 ft. lbs. when aligned.

Wrench used was a Brownells combination wrench, set at 90 degrees from the axis of the various extensions used, See:

http://www.brownells.com/gunsmith-tools-supplies/rifle-tools/wrenches/ar-15-m16-combination-wrench-prod468.aspx

10

Disassemble the upper and loosen the barrel nut. Set your torque wrench (if you feel you need it) to the minimum required value and torque it. Then apply whatever force you need to get the nut to line up if it didn’t do so.

11

Are you adjusting your cheek weld during groups? If so, keep a solid cheek throughout each group and this will minimize parallax.

Also check that your optics are ‘loaded forward’. The Larue mounts usually dont allow for much wiggle room, but they should be pressed forward (toward the muzzle) as you clamp them down. The mount/scope is wanting to move forward under recoil. Same goes for the optic ‘loaded forward’ to the mount if it mounts with a cross-bar (ACOGS dont IIRC).

12

As most people speculated/assessed, it was probably an assembly problem.

I tore down the carbine a few days ago, and immediately put it back together. I did the old “snug up three times” process, then set my wrench for 30 ft. lbs. and torqued. Not aligned for gas tube. Set the wrench to 40 ft. lbs., torqued, and it was aligned. Finished assembly.

I also tightened the mount/scope connection. It wasn’t exactly loose, but would tighten about 1/8 turn.

Groups have shrunk by about 1/3rd (about 2" at 50 yards with a 1.5x ACOG and federal XM193). Groups with irons are the same (my usual). No wandering zero or shift in PoI.

It just goes to show that I need some more experience assisting others to do a build.

13

Thanks for the update. As I have mentioned before if you don’t want to be chasing torque all day, simply set it to 30 ft./lbs. and then leave it. If it doesn’t align at that point then apply steady pressure until it lines up and stop. The chances of you going to 80 are pretty slim IME.