Is this normal?

[Rgarbarino]

I put a scope on my AR and got it pretty dialed in. Then I changed to a full floating rail set up. Did all the work myself the right way with a vice block, armorers wrench, torque wrench, etc. Clamped the barrel when removing/installing the muzzle break. Had to go to about 70lbs on the barrel nut to get the gas tube hole aligned.

Took it shooting again yesterday and the first few shots at 100 yards were not even on the paper, about 18" low and 12" right. After about 20 rounds I got it dialed back in.

I assume it's normal for a scope to be off by that much when you make the changes I did and removing and re-installing a scope?

[jimmykan]

That's not too hard to believe...

18" low and 12" to the right, means a total straight line deviation of 21.633" at 100 yards.

21.633 inches = 0.611 yards

arctan(0.611 yards / 100 yards) = 0.350 degrees.

So the question comes down to:

Is it possible that when something gets unscrewed, and then re-tightened to a different torque setting, that the alignment could change by 0.350 degrees?

I think so.

From a machinist's point of view, it's not uncommon for tapped threads to be more than a degree or two off-axis.

Even single-point cut threads are subject to fit tolerances. The shorter the threaded length, the more off-axis deviation you might see.

This is why many gunsmiths who accurize AR rifles offer truing of the upper receiver's front face.

[SFCRangerDoc]

Yeah this totally makes sense...even adding something as simple as a muzzle brake/FH will change the harmonics of the barrel and change POI. You've gone quite a bit farther than that and as jimmykan stated theres a lot of other factors in there as well. I would say as long as things stay dialed in from here on out you'll be fine.

[BlueFalcon]

That's not too hard to believe...

18" low and 12" to the right, means a total straight line deviation of 21.633" at 100 yards.

21.633 inches = 0.611 yards

arctan(0.611 yards / 100 yards) = 0.350 degrees.

So the question comes down to:

Is it possible that when something gets unscrewed, and then re-tightened to a different torque setting, that the alignment could change by 0.350 degrees?

I think so.

From a machinist's point of view, it's not uncommon for tapped threads to be more than a degree or two off-axis.

Even single-point cut threads are subject to fit tolerances. The shorter the threaded length, the more off-axis deviation you might see.

This is why many gunsmiths who accurize AR rifles offer truing of the upper receiver's front face.

mind blown.....

[bsumoba]

lots of things could be happening to you.

what scope and rings do you have? if its a barska or similarly priced scope or cheap rings, you could simply have issues when reinstalling the scope and rings back onto your rifle. Did you set it up on the same pic rail locations as well?

Second, like ^ person stated, when removing and reinstalling the barrel, the barrel itself might not be perfectly concentric or the action face could possible not be perpendicular to the bore axis. Couple these two things and at 100 yards, the deviation could be huge.

What barrel do you have or what mfg was the upper? Factory barrels and actions are notorious for having this kind of problem. Example, Remington 700 actions are cranked out like crazy and there are manufacturing variances in it and many people send out their actions to get them "blueprinted" or trued up in order to get maximum accuracy out of them. If the chambering was done in a barrel that was not setup properly on the lathe, the chamber could have been cut such that it is not concentric or perpendicular to the bore axis.

its just the nature of buying factory stock.

[Rgarbarino]

The rifle is a Colt 6920/5.56 completely factory stock upper/barrel. The scope is a Nikon M223 2x8x32 on a Nikon M223 20 MOA mount.

[dtrump]

As others have stated all the factors in changing over to free float can cause poi shift, coupled with remounting the optic this is very common. As long as when you make your adjustments in your optic and it continues to be accurate from there, you are all set. I doubt it will continue to shift point of impact from there.

[russ69]

...Is it possible that when something gets unscrewed, and then re-tightened to a different torque setting, that the alignment could change by 0.350 degrees? I think so...

Nice work Jimmykan. Yes it's amazing how little things can have such a big affect but it happens all the time. I have seen 6 inch changes at 100 yards from just switching ammo. I'm not sure if any math equation can sort that out but I've seen it more than once.

[Arnelcheeze]

You did remove everything so a shift should be normal, but from my experience,
I would swap out those nikon rings, they are not the best out there.

[L4D]

its easy to be that guy at the range with the adjustment caps off, a handful of allen wrenches, no rest....and never get your POI dialed in. ask me how I know.

[bsumoba]

its easy to be that guy at the range with the adjustment caps off, a handful of allen wrenches, no rest....and never get your POI dialed in. ask me how I know.

dont forget the rear support on the rifle. i see a lot of guys shooting with a stable front rest, but are free floating the buttstock on their shoulder with no rear bag.

[nate76239]

I have a Nikon p223 4-12x40 with the m223 mount linked below and I'm sure the issue is the mount not the rifle. There can be too much play from right to left before you clamp it back down. Ran into that issue at a range as soon as I got home and got the bore sight on I could see it was off before I clamped it down

http://www.midwayusa.com/product/373994/nikon-m-223-1-piece-scope-mount-picatinny-style-integral-1-rings-flat-top-ar-15-matte

[bsumoba]

I have a Nikon p223 4-12x40 with the m223 mount linked below and I'm sure the issue is the mount not the rifle. There can be too much play from right to left before you clamp it back down. Ran into that issue at a range as soon as I got home and got the bore sight on I could see it was off before I clamped it down
http://www.midwayusa.com/product/373...op-ar-15-matte

why not take the upper off, lay in on the bench (upper only), take bcg out, then aim the bore at a target, then adjust your scope to wherever your bore is aimed at to get a rough site in? no need to go home

attributing this POI shift to the mount would not account for this elevation difference, unless he really did a hack job on the mounting. i could see up to 5 MOA in change, but 1-2 (12-24 inches) feet of elevation and windage correction is a lot to blame on the mount. that has to be one crappy mount.

its primarily him taking off the barrel and re-installing it.

[TheExiled]

That's not too hard to believe...

18" low and 12" to the right, means a total straight line deviation of 21.633" at 100 yards.

21.633 inches = 0.611 yards

arctan(0.611 yards / 100 yards) = 0.350 degrees.

So the question comes down to:

Is it possible that when something gets unscrewed, and then re-tightened to a different torque setting, that the alignment could change by 0.350 degrees?

I think so.

From a machinist's point of view, it's not uncommon for tapped threads to be more than a degree or two off-axis.

Even single-point cut threads are subject to fit tolerances. The shorter the threaded length, the more off-axis deviation you might see.

This is why many gunsmiths who accurize AR rifles offer truing of the upper receiver's front face.

[Rgarbarino]

As I continue to shoot I'll keep an eye on how well it holds zero. I'm thinking it will be just fine. One thing I did just notice is that there seems to be two types of bases on the M223 mount, a solid 1 piece with 2 mounting bolts and a version with separate side clamps with 3 bolts. Mine is the later with 3 bolts and side clamps. Both pictured here.