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Posted: 1/14/2015 4:43:38 PM EST
I am new to the AR platform having been a bolt action guy. I mounted a Vortex Viper PST on a Primary Weapons System gun using Bobro two lever mounts. During zeroing this setup required 20 minutes of angle right adjustment to zero at 100 yards. Twenty minutes would be unacceptable on a bolt gun but I was told this is an acceptable tolerance on an AR. The scope mount is solidly on the receiver rail with ring screws properly torqued. Does anyone have an opinion on this?
Link Posted: 1/14/2015 4:59:42 PM EST
Scope may not have been at mechanical zero when you purchased it.
Link Posted: 1/14/2015 6:22:19 PM EST
[Last Edit: 1/14/2015 6:23:04 PM EST by MS556]
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Originally Posted By Synyster06Gates:
Scope may not have been at mechanical zero when you purchased it.
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I know you don't want to re-zero, but I'd like that question answered too. Was the scope windage mechanically centered when mounted? If it was and you had to dial in that much windage to zero it, something is wrong. I doubt it is the Bobro, but anything is possible.
Link Posted: 1/14/2015 6:32:16 PM EST
Milspec doesn't necessarily mean In-spec.
Link Posted: 1/14/2015 6:34:59 PM EST
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Originally Posted By RFutch:
Milspec doesn't necessarily mean In-spec.
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Maybe I missed it... Where did anyone mention milspec
Link Posted: 1/14/2015 6:40:31 PM EST
[Last Edit: 1/14/2015 6:46:39 PM EST by MS556]
Perhaps the assumption of OP is that the receiver upper and his barrel are exactly parallel to each other. Regrettably, they sometimes are not very close. There are fixes for this, if it is his problem. If the receiver upper rail slots are actually out of spec, that is a different can of worms. But, first we need to know whether the scope windage adjustment was centered when he mounted the scope. He also could have over torqued a scope ring. Lots of possibilities. But 20 MOA of lost windage in one direction is not something I would accept on a scope.

Link Posted: 1/14/2015 6:44:31 PM EST
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Originally Posted By Synyster06Gates:


Maybe I missed it... Where did anyone mention milspec
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Originally Posted By Synyster06Gates:
Originally Posted By RFutch:
Milspec doesn't necessarily mean In-spec.


Maybe I missed it... Where did anyone mention milspec


No one did. But the term is thrown around so much in the industry to cover all kinds of crap.

It was not meant to point blame at anyone in particular. Carry on...
Link Posted: 1/14/2015 8:35:29 PM EST
20 MOA is less than two feet at 100y. I wouldn't loose much sleep over that as long as it zeros and you have some adjustment left over.

Tolerance stacking makes it really hard to assign blame to one part or another. Barrel could shoot left. Upper to barrel could be not quite square (some guys true their upper receivers before mounting a barrel), upper rails might not be quite true, scope mount to rail, rings to scope, and finally the scope itself.

It would be an interesting test to mount some irons on that upper and see how far left/right of center they end up.

Either way I don't think that less than 2 feet is much to be worried about since it zeroed in the end, unless you think you'll be dialing both wind and elevation at the limits of your scopes adjustment range (800y and dialing for wind?). In which case a mount with some built in cant would put you back into safe territory.
Link Posted: 1/15/2015 7:42:35 AM EST
I would try another set of rings/mount before going any further. That would rule out the current mount.
Link Posted: 1/15/2015 8:01:18 AM EST
See how much the BUIS needs to move, if any.
Link Posted: 1/15/2015 8:30:19 AM EST
[Last Edit: 1/15/2015 8:31:37 AM EST by Number1gun]
My guess would be you ended up with a bit of cant when the base levers were closed and they are binding somewhere at the rail/base. It does not take much. You are working with Aluminum..! Lever adjustment may be too tight, burr from manufacturing, etc..
If it was scope to rings you would have noticed right away that it was not sitting in the rings correctly before the top half was installed.
Other option is of course...dissemble compleatly and start over double checking everything from the rail up and see what you get ?



Link Posted: 1/15/2015 10:41:55 AM EST
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Originally Posted By Chrome308:
20 MOA is less than two feet at 100y. I wouldn't loose much sleep over that as long as it zeros and you have some adjustment left over.

Tolerance stacking makes it really hard to assign blame to one part or another. Barrel could shoot left. Upper to barrel could be not quite square (some guys true their upper receivers before mounting a barrel), upper rails might not be quite true, scope mount to rail, rings to scope, and finally the scope itself.

It would be an interesting test to mount some irons on that upper and see how far left/right of center they end up.

Either way I don't think that less than 2 feet is much to be worried about since it zeroed in the end, unless you think you'll be dialing both wind and elevation at the limits of your scopes adjustment range (800y and dialing for wind?). In which case a mount with some built in cant would put you back into safe territory.
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Wise observation. Unless shooting very long range, probably will not matter. Testing with the rear iron zero would tell alot. If the windage ends up way off, you know it is not the scope and mount.
Link Posted: 1/19/2015 2:51:50 PM EST
Want The Solution?

If you got a boresighter with a collimator grid/scale you are set; otherwise lots of click counting...


Need to pull the scope from your QD mount and mount it in standard rings. You want to see if it is the QD mount that caused the lateral shift. Damn sure is important to keep as much vertical zero as you can when reticle is at theoretical center. You lose LOTS of vertical range if the reticle is at extreme windage position. If you got 65moa to total movement range, losing 20moa means you only got 12moa windage to that side remaining. The reticle won't move its full 65moa range either, even if you got a 40moa canted rail which gives you a 200yd zero with reticle all the way up in the vertical.

Was using LaRue QD risers on 3 rifles for many years. Swapped scope to another AR w/o a riser and saw a 20moa LH offset on the boresighter grid (Bushnell 74-3333). Put the old scope back on to verify zero position on the grid. Put it on the LaRue and wow! 20moa off! Tried same test w/another scope using the LaRue mount (L-110?) and same thing. Not to go into the saga of contacting LaRue, but no joy there...

No longer use ANY riser or QD side-lever type mount.

To center your scope (theoretical) using collimator grid, you split the difference using the grid and divide by half, then move the reticle back to the halfway point, not the middle grid mark, halfway in total MOA range. If reticle is past grid marks on one side/both, you count the # clicks past the grid on both sides add those to the total grid number and divide by half. Then dial your scope both ways to be sure reticle rests on same grid mark each time. No grid? Dial all windage to one side, then count clicks all the way to the other side; split in half then.

Colimator/bore-sighter gives you a reference to work from and can diagnose scope problems as well as align reticle for installation very quickly. Also can use to record zeros in log, just be sure to note which rail slots used as well and not shift scope in the rings....

Link Posted: 1/20/2015 11:09:20 AM EST
Thanks for the replies guys. Here is my course of action when weather and time permit. I have already removed and replaced the scope from the mount to check for problems. Rings retorqued to 15-18 inch-pounds as recommended by Bobro. I will compare required windage on BUIS at 25, 50, and 100 yards. I will check mechanical zero on scope. And I will be able to try the scope/mount assembly on another AR next month.
Link Posted: 1/20/2015 12:46:49 PM EST
Let us know if the windage is off to the side on your BUIS. That will speak volumes and probably should be the first thing and easiest thing to do, when you find time.
Link Posted: 1/21/2015 9:06:55 AM EST
Mystery solved. The Vortex Viper PST scope comes out of the box with the windage indicator at zero. This is not optically centered but at full left windage. From here the only adjustment to zero the point of impact is to add right windage. 20 MOA right windage puts me near the center of the optic center of the scope. If this information had been included in the owner's manual or web site this confusion could have been avoided. Thanks for all of your valuable input and advise.
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