Shim thickness?

[M-61]

If you want to increase the height of a scope mount and rails (10-20-30moa) are not available what should the thickness of a shim be for 20MOA?

 

[gaboon]

Beer can.

 

[Erik Cortina]

I believe it's pretty close to .001" per MOA.

 

[dmoran]

It is dependent on the length of the rings.

Spacing Shim Needed
(inch) (per MOA)

3 0.0009
3.5 0.0010
4 0.0012
4.5 0.0013
5 0.0015
5.5 0.0016
6 0.0017
6.5 0.0019
7 0.0020
7.5 0.0022
8 0.0023
8.5 0.0025
9 0.0026
9.5 0.0028
10 0.0029

Formula =
Length-Spacing X 2
X Pi (3.14159)
divided by 360-degrees (360)
divided by 60-arcminute (60)

Example = for a 5" Spacing
5 x 2 x 3.14159 / 360 / 60 = 0.0015" of shim per MOA

 

[CatShooter]

IT is dependent on the spacing between the attachment screws, not the total length of the rails.

If a 20" rail is attached to a rem 700, the measurement is taken between the mounting screw holes (~4-ish inches), not the 20" of rail.

 

[dmoran]

IT is dependent on the spacing between the attachment screws, not the total length of the rails.

If a 20" rail is attached to a rem 700, the measurement is taken between the mounting screw holes (~4-ish inches), not the 20" of rail.

It is dependent on the spacing of the Rings.....

 

[gotcha]

+1 ...... You might want to consider the Burris signature Z rings. They have a shim kit available to yield 10, 15 & 20MOA for one inch tubes and a set of 20MOA shims for 30 MM tubes. The actual MOA you achieve is interdependent, as DMORAN says, on ring spacing. Just a thought.

 

[22BRGUY]

It is dependent on the length of the rail.

Spacing Shim Needed
(inch) (per MOA)

3 0.0009
3.5 0.0010
4 0.0012
4.5 0.0013
5 0.0015
5.5 0.0016
6 0.0017
6.5 0.0019
7 0.0020
7.5 0.0022
8 0.0023
8.5 0.0025
9 0.0026
9.5 0.0028
10 0.0029

Formula =
Length-Spacing X 2
X Pi (3.14159)
divided by 360-degrees (360)
divided by 60-arcminute (60)

Example = for a 5" Spacing
5 x 2 x 3.14159 / 360 / 60 = 0.0015" of shim per MOA

Great info as always! Thanks Donovan

 

[M-61]

Fabulous replies. There was NO way I was figuring this out myself....ever.

BUT....I understand what I need thanks to the replies, now the BUT. ( which came from the replies) If you buy say a 20MOA mount....how can they say it's 20MOA if they have no idea of the mounting distance of the rings?
Or by thinking of this have I added more confusion to my thoughts?

 

[CatShooter]

Fabulous replies. There was NO way I was figuring this out myself....ever.

BUT....I understand what I need thanks to the replies, now the BUT. ( which came from the replies) If you buy say a 20MOA mount....how can they say it's 20MOA if they have no idea of the mounting distance of the rings?
Or by thinking of this have I added more confusion to my thoughts?

When a mount or rail is spec'ed as a "20 MOA mount", it means that the underside of the front screw mounting point has been machined ~20 thou (0.020") thinner than their standard mount.

It is the spacing of the attachment points, not the spacing of the rings.

When you mount a rail parallel to the receiver, and then have rings that are in equal height, to mount the scope - whatever spacing you have with the rings, the scope will be parallel to the receiver.

If you put a 20 mil shim on the rear receiver bridge (which is where shims are put) it willl tilt the rail down by 20 MOA - it will make no difference how far the ringe are spaced.

If you use Burris Signature rings, this relationship changes somewhat - but you still do not have a lot of latitude in ring spacing, maybe +/- 1"

For normal set ups, even on long rails, a good rule of thumb is 1 thou is = to 1 MOA

 

[BoydAllen]

I disagree. 20 MOA (in shooting, minute of angle, in math, minute of arc) is a third of a degree (60 minutes per degree, 60 seconds per minute) and the taper in the chart in the previous post is right. Recently, I went over this with a friend, and did a little figuring. Based on a distance between support points of 5", the back would be elevated .029 (or the front lowered by the same amount) to achieve a 20 MOA tilt. Obviously, rails that have closer mounting point spacings would require thinner shims than those with longer.

 

[gotcha]

Thanks Boyd, Base mount holes will vary LA to SA also between Mfgs. Since bases are designed to hug the contour of the action it would seem that lifting the rear of the base with a shim would possibly nullify the advantage of contoured contact and make the base less stable. Also, in my mind, there is a chance of warping (dishing) the base when torqueing the base mount screws. I'm referring to one piece steel or aluminum bases. Any thoughts?

 

[BoydAllen]

Epoxy bed the base at both ends, with the screws only slightly tightened, not enough to distort, just to position. You could use a wire shim with release, and put release on the action, and then tighten the base after the bedding set up, Devcon plastic Steel or the like.

 

[dkhunt14]

Good answer Boyd. It will bend if you don't. We did it another way. We went to 1000 yards and used a feeler gauge to see how much we needed to put the scope in the middle and be on at 1000. We shot and tested it. When you put the feeler gauge between the screws in the back and tried to tighten it, the little bit of pressure bowed the base. We just wanted to see how much so we could mill a new base. Matt

 

[gotcha]

Boyd & Duckhunt, Thanks for the input.

 

[yenak]

FWIW I just got done shimming a SA rifle. As Catshooter said .001/1 MOA