Chasing zero gremlins

[gatordev]

So here's a run down of what I've done so far and I'm hoping someone can point me in a direction that may address the issue.

I took a stock 6920, took off the barrel and replaced it with a Colt 0921 barrel (brand new) and converted the rifle into a basic SOPMOD Block II configuration. I did this in December and took great care in making sure the rail was installed straight and true per the instructions. The only real issue I had was a stubborn barrel nut, but as mentioned in another thread, I believe that issue was overcome. So here's the breakdown of what happened...

-Zeroed the RDS (Eotech) in December (cool and rainy) with Federal XM193 for a 50m zero. Didn't have time to zero the iron sights.

-Moved back to the same area and finally had a chance to take it out and finish/confirm the zero.

-Eotech was shooting 4 inches (!) to the right of zero with Privi .223. Weird, but after an adjustment, it's shooting to zero.

-Zeroed the iron sights (Matech rear that came with the rifle and has always been fine and KAC front). Not even on the paper. Adjusted Matech to the stop and still shooting several inches right.

-Tried XM193 just to make sure there wasn't some weird ammo issue...not that it would cause a 4" disparity. XM193 is pretty close to the Prvi stuff. Damn.

-Took off and reinstalled the front sight. It helped a bit, but not enough. The rear is still at the stop and not zeroed.

-Triple checked the rail alignment, both rotational (good) and lateral (see pics...all testing was done without the can, despite the pic). Everything looks both concentric and even.

http://i840.photobucket.com/albums/zz322/gatordev/Public%20pics/DSC02127_zps68ed12f1.jpg

http://i840.photobucket.com/albums/zz322/gatordev/Public%20pics/DSC02125_zps66af2714.jpg

Normally I'd assume a crooked rail (and it may still be that), but since the Eotech was out initially too, I'm really scratching my head. Throughout the whole process, the rifle has shot consistently ie, the POI isn't dancing all over the place...they're very consistent. Obviously something is out of alignment, but I can't figure out any other way to "test" what that might be since the rail (the usual culprit) looks concentric/even.

Thoughts?

[GOST]

Is it's groups tight?

[gatordev]

For a 55 gr bullet and a Colt barrel, yes. I have a feeling the barrel isn't a huge fan of the Prvi, but it's still an average group for someone not trying hard with a less than optimal support. Like I said, they were consistent and together, but just landing in BFE.

[GOST]

Almost sounds like the barrel isn't seated good. Have you tried this barrel on another upper yet?

[gatordev]

I'm not sure how it couldn't be seated, since the nut was tightened and loosened several times, but never say never, I guess. If you've ever had to install a RIS II, you'd know why I'm not excited about taking the barrel off. All those screws are a pain. That said, I'm thinking it's not the barrel...

I did a little more measuring and it looks like the rail is "pointing" slightly to the right (relative to the barrel) by a smidge under 1/16". There's a larger gap between the right side of the barrel and the right side of the rail by that much compared to the left side.

This is the part that makes my head hurt though, so please correct me if I'm thinking this backwards. Assuming everything is relative to the centerline of the barrel, if the rail centerline is aligned to the right, then with the sights centered, wouldn't that make the rounds hit left (barrel CL would be left of rail centerline which would be centered on target)?

Or do I have this backwards, in which case maybe that slightly less than 1/16" is enough to hit the rear sight stop?

Man I really don't want to have to deal with those screws again....

[GOST]

Your reasoning sounds right to me, but if the RDS is mounted to the receiver the rail shouldn't affect it.

[GOST]

It's a shame that your having those problems with such a nice build.

[GOST]

There is the possibility that the receiver face that interfaces with the barrel extension is flat and true. I know it's a pain but trying the barrel on another receiver could tell you a lot.

[FortTom]

GD, try this, maybe an easy way out.
Go to the hardware store and get one of those short squares, 90 deg., that has both a vertical and horizontal bubble level In it. Put your rifle in a vise and square it up at the receiver. Top should be level front to rear, and side to side.

Then place the top of the square, it should have an aluminum mini-level on top, and a rule facing 90 deg. down. With your receiver square, now hang the square off the top of the rail (push into the outside of the rail slots, and inside noting the bubble's location. It should be darn near centered.

Your square should have one horizontal bubble level, and one 45 deg in the corner where the rule inserts into the metal top. The bubble should now be centered in that, or somewhere very close. It doesn't have to be perfect but pretty close, a degree can make a big difference in the bubble's location. Am I making sense?

O.K. last thing, rotate the rifle left or right in your vise, 90 deg., and level the receiver in some rags. This might be a pain in the butt, you'll have to get it level on your horizontal plane, like your artificial horizon and front to rear. So now your rail should look like an inverted "V", now. Something like this /\. Cradle the level on the corner of your rail, which is now at 12:00.
The corner level bubble should actually be at 90 degrees now. Look and see where bubble is.

If you haven't run out of patience by then, you can repeat step one and two, and you should find out if your rail is twisted or true on both the horizontal and vertical planes.

If not you might have to buy some shim material in .001" increments, cut pieces as appropriate, and insert them opposite of the direction of your bubble and square things up. I've pulled this stunt off once before for a friend, and discovered that under recoil the gas block was touching the top of the rail momentarily until the rail was eventually knocked out of "square".

Hopefully nothing that drastic will happen. If you find it's not square one way or another, the easy fix might just be loosening it up and re-installing with your shim stock and a third hand available.

If you do nothing, you'll at least know if your rail is "twisted". I'd just take the simple readings off the square before I did anything more drastic than just loosening up the rail and re-tightening the rail.

I have to believe that something on the rail is loose or has been knocked out of alignment, rotated, having read your posts, especially if I read them right and the top of your receiver levels correctly top dead center and concentric with the center line of the barrel. I hope this helps. If not you can always use your new level to hang the pictures for the wife....

If it works, have a beer.

If everything is FUBAR, say screw it and have several. Then a day or so later, devise a plan B. Hope this helps.

FT

[DMViergever]

Which handguard is that?

[jjl103]

Which handguard is that?

DD RIS II?

[FortTom]

Also, did you check under the scope mount "clamps" to ensure that there is no foreign matter in there, or that they are not damaged. Also the rails, where they are connected? No dings or a small chip or something causing your mount to consistently mount, but at a few degrees off from the center of your mount?

FT.

[gatordev]

There is the possibility that the receiver face that interfaces with the barrel extension is flat and true. I know it's a pain but trying the barrel on another receiver could tell you a lot.

I'm guessing you meant "isn't flat and true?" The receiver is the Colt receiver that came with the original complete 6920. I've probably put over 500 rounds through that factory rifle with zero aiming issues, so I don't believe it's the receiver itself. I guess something could be slightly off with the barrel extension since it's not the original barrel that came with the rifle, but I find that hard to believe since it's a factory barrel. Never say never, I guess (again).


Also, did you check under the scope mount "clamps" to ensure that there is no foreign matter in there, or that they are not damaged. Also the rails, where they are connected? No dings or a small chip or something causing your mount to consistently mount, but at a few degrees off from the center of your mount?

FT.

The Eotech is mounted on the receiver, though, and it's shooting straight (after an adjustment), so the issue would be the sight on the rail and there's nothing in there. I bought the rail used many months ago when I started collecting parts, so I guess it could be warped, but I just have a hard time believing that looking at it. I'm thinking, despite your trigonometric plan, the easier option is to just redo the rail install and see what happens. If that's not solving it, since the RDS works fine, I'll probably just move on and take the irons off. It's not like I can see that front sight much past 50 m anyway. Aging sucks.

[FortTom]

Ha..ha... yeah my BUIS's are just on there for looks...maybe the bad guy will actually think I can see him....[:D]

FT....

[gatordev]

I have to say, I'm a big fan of KAC irons if the name of the game is precision. The skinny front post coupled with the small rear aperture really work for me and was able to hit clays at 200 yards in the past. I'm not sure I could do that with a set of Troys. But beyond plinking like that, it's just easier to run a RDS at this point.

Didn't even bother to mess with this rifle today and instead went and blew through a bunch of 7n6...because I can.

[GOST]

Didn't even bother to mess with my rifles today instead went to work cause they made me.