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Old February 11, 2015, 02:31 AM   #1
Koda94
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how well does pictanny rail hold zero?

Ive never considered this before but with the ease of clamping sights on a pictanny rail the thought hit me one could possibly swap out different optics or sights providing the sights returned to zero.

lets say you have a pictanny rail mounted scope, zeroed at 300 yards. If you remove the scope and re-install it in the same position how well does it hold its sight in?
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Old February 11, 2015, 02:38 AM   #2
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It aint because you can not line it up in the precise position as it was before.
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Old February 11, 2015, 03:35 AM   #3
FrankenMauser
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I have swapped scopes quite a few times on picatinny rails, and return to zero is mostly a matter of mount quality and technique. I have never had a perfect return to zero, though. (Within 1-2 MoA, but that's about as good as it gets with my equipment.)

If you're using a crappy mount that doesn't index solidly on the rail, return to zero will not be great.

If you don't use consistent technique, return to zero will not be great.


I have even had a scope go back and forth between a trio of ARs, and the zero was within 2-3 inches @ 100 yards, across the lot of them.

As far as technique goes...
All of the optics that I have swapped back and forth* have had the fixed side of their mount on the left side, and the clamping jaw on the right side.
So, by setting the mount on the rail and pushing down, forward, and right, while snugging down the clamping hardware, return to zero has been excellent.
That pushes the mount down against the rail, to the front of the slots in the rail (where recoil will try to force them, later), and against one side of the rail.
Finishing by tightening the screws/nuts to the same torque also helps.



*I have also swapped a scope wearing Millet windage-adjustable rings, but I went into that situation knowing full well that there was no hope for a return to zero. Without fixing the clamps on one side of the rings, there is no way to lock them in place for repeatability.
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Old February 11, 2015, 04:25 AM   #4
nemesiss45
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It all depends on the quality of the mounts. Get high end mounts from a good manufacturer and they should be close. A camming lever will be best.as it should give you the same torque on ths mount. Thumb screws will pull to one side unless the torque is perfect.
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Old February 11, 2015, 10:18 AM   #5
jmr40
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I've done so with both rails and Weaver type rings and bases. The key is proper technique. When you initially mount the scope slide the rings as far forward as possible then tighten the rings. During recoil they want to move forward anyway, if they are already as far as they can go you'll get no movement. If you remove it, slide it as far forward as possible when re-mounting.

I've never had to do anymore than minor tweaking of the scope adjustments after. Certainly not off enough to cause a miss on deer size game at reasonable ranges.
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Old February 11, 2015, 02:20 PM   #6
Panfisher
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I swap scopes back and forth on my R15. As stated technique is a big part of it. Push it forward before you tighten it down. When I started it took a few times on and off before it started holding zero pretty well. Not well enough I would have wanted to shoot a match or anything, but certain not off enough to cause me any lost sleep for coyotes, deer, or pop cans.
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Old February 11, 2015, 03:11 PM   #7
Bart B.
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I've removed, replaced, swapped scopes on rail for decades. They all return to set zeros with my Weaver or Tasco rings with 1/4 MOA error, at most. All with thumb or coin screws torqued to some amount. There's a bigger difference in atmospheric conditons to think about.
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Old February 11, 2015, 03:29 PM   #8
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yes, if your mounts are fitted nicely and lock in at the same position, shouldn't be an issue. I would assume a nice one-piece mount would be ideal for changing scopes around a lot.
http://www.galatiinternational.com/product/MD110.html
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Old February 11, 2015, 10:35 PM   #9
reynolds357
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Depends on who makes the rail and who makes the rings. It also depends on how you install them.
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Old February 11, 2015, 10:53 PM   #10
EchoM70
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Quote:
Originally Posted by nemesis45
It all depends on the quality of the mounts. Get high end mounts from a good manufacturer and they should be close. A camming lever will be best.as it should give you the same torque on ths mount. Thumb screws will pull to one side unless the torque is perfect.
+1

I've went exclusively to Bobro mounts on my AR's. They're the best at returning to zero that I've experienced. Usually spot on.
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Old February 12, 2015, 02:25 AM   #11
bamaranger
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works for me

I've got a set of Leupold QR Levers (rings) on a Savage Scout, and the scope has been on and off numerous times. The Savage scout rail appears aluminum. The rifle comes back to within 1MOA or less, every time, so much so that I don't worry about it.

I changed slots, and THAT did effect POI enough that I had to rezero.
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Old February 12, 2015, 05:42 PM   #12
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I use Warne QR rings for my takedown Browning BRL, I got within 1 MOA repeatability on a 4 series test (initial and three remount). As the gun groups 2 MOA it was basically only a slight shift within an overall 3" grouping.
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Old February 13, 2015, 06:59 AM   #13
gman3
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I have a couple of Warne's, close, but not exactly. I have a couple of Badgers, they are about dead on every time if I take the scope off and put it back.
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Old February 13, 2015, 11:45 PM   #14
HiBC
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To begin,in my head from draft angle days,a degree is about .017+ thousandth of an inch,per inch.

Just to illustrate the math,doing it in our head,lets call it .016 thou per in.

Now,just for this example,lets make the ring spacing 4 inches.

What would a .001 lateral or vertical error do to POI?

Well,.016 per in would be .064 over 4 in for 1 deg.As there are 60 MOA per deg,64 thousandths would translate roughly to 1 MOA per .001 error in ring to rail variation.

The design of the picatinny rail is quite similar to machine shop fixturing.

If the surfaces are accurately machined,clean,and have no burrs ,dings,casting roughness,etc they should repeat postioning to way less than .001 .That is a big "IF".

There is another issue.Alignment.If the scope rings are not perfectly clocked to the same orientation on the tube,tightening the rings to the base makes the scope tube a torsion bar.That introduces variability.Tightening the ring clamp screws can easily introduce this error.

Two piece bases?.Unless you have set up on a granite surface plate with a test indicator and confirmed alignments,do not assume the front and rear bases and the rings will all come into alignment.If they are not coaxial,tightening the rings to the bases involves flexing the scope tube.

This "springy" stuff makes repetition tough.

I thinkif you have a flat,true,rigid rail,such as on a quality AR upper,and you use a quality one piece ringmount,that is precisely CNC machined,
so that clean,non stressed surfaces can come together,repeatition will be very close..Should be easily sub MOA.

Last edited by HiBC; February 13, 2015 at 11:50 PM.
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Old February 14, 2015, 01:22 AM   #15
Koda94
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Some good information, it sounds like if one expects to interchange optics then a well machined 1pc mount is the way to go
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Old February 14, 2015, 01:29 AM   #16
FrankenMauser
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You guys with your sub-MoA talk are making me regret never measuring the actual deviation when re-mounting the scopes on my ARs. Next time I'm out, I'll have to see if I mis-remembered the "2-3" @ 100 yards" and the return to zero is better than that, or if I'm not losing my memory, after all.
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Old February 14, 2015, 02:04 AM   #17
4thPoint
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Quote:
Some good information, it sounds like if one expects to interchange optics then a well machined 1pc mount is the way to go
It's not the Picatinny rail that will prevent return-to-zero (assuming it's well manufactured) since it's a passive piece. The mounts will have a Much greater affect, choose wisely.
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Old February 14, 2015, 06:57 AM   #18
Mobuck
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"A camming lever will be best.as it should give you the same torque on ths mount. Thumb screws will pull to one side unless the torque is perfect."

Better check this? While a cam may create a repeatable clamping force, most also act against only one side of the mount-same as a screw. If a throw lever mount acts on BOTH sides of the rail, it most certainly WILL NOT be as repeatable as a screw clamp with one solid side.
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Old February 14, 2015, 08:24 AM   #19
Bart B.
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To make simple/easy angular error calculations easy for two things some length apart:

* 1/3600 of the length equals 1 MOA.

* 1/60th of the length equals 1 DOA (degree of angle).

That fits the USA shooting sports '1 MOA = 1 inch per hundred yards' standard.
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Last edited by Bart B.; February 14, 2015 at 08:33 AM.
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