request for formula for scope shim - The Firearms Forum

B&E Detective · 09-03-2007, 08:46 PM

Does anyone have a formula as to shim thickness to raise my front scope mount approximately 4 inches higher from 75 to 100 yards. At the top of my scope travel I'm still 4 inches low at 100 yards. I placed a coke can sliver in the front mount which is .005" thickness, but I'd like a formula to figure out correct thickness to correct this problem.

strayshot · 09-03-2007, 09:56 PM

I don't know the shim thickness and there are more experienced folks on here then me but, If you're saying the bullet hole is 4" lower than the crosshairs, than you need to move the scope down...unless I'm not understanding here.

Don Buckbee · 09-04-2007, 06:44 AM

Burris Signature rings, the ones with the adjustable inserts, are a far better solution to getting a scope zeroed than shimming. Shimming one ring higher puts a stress on the scope tube, which is not good.

Don

LDBennett · 09-04-2007, 08:50 AM

Its a ratio thing. You are comparing how far you want to move the bullet on the target with its shooting range and how much shimming required over the length between the mounting points on the gun.

(bullet errror in inches) x (receiver mounting distance in inches) divided by (range in INCHES) = shim thickness in inches

Note all measurement are in the same measuring units, inches in this case.

If it is shooting low now then you want to elevate the rear of the scope to make the scope look where the bullet is going. You'll need more than the calculated shim because you don't want to end up eith the scope at the extreme end of its adjustment as you now have it. Doubling the above calculation results might get you closer to the ideal.

If you have to use too big a shim you are indeed going to stress the scope tube. The Burris plastic insert mounts are the real answer, not shims, if the required shim stock is too much. When you use thick shim stock you move the stress of the recoil to the screws as the mounts are not down on the receiver and the friction of that juncture is not assisting resisting the recoil. The screws see more of the recoil forces and may snap off if the shimming is too extreme. The Burris mounts eliminate that problem and look neater too. Run through the above calculation and review the Burris application information to see what set of offsets you need. I'm not familiar with how they spec them but I would think the instructions that come with them would be helpfull. You can use a bore sighting laser tool to help in all of this.

LDBennett

B&E Detective · 09-04-2007, 08:33 PM

I appreciate all the answers to this problem. I'm looking at and ordering a set of Burris Signature rings this evening. Thanks again.

ohsix · 10-09-2007, 08:00 PM

Distance in Yards Shim Thickness Point of impact in inches
25 .001 .250
50 .001 .500
100 .001 1.0
200 .001 2.0

25 .005 1.25
50 .005 2.5
100 .005 5.0
200 .005 10.0

When the rings are 3.25 inches spacing.

I would try a .004 shim or a .006 shim under the rear base and see if U can adjust the scope.When U raise the rear base the POI goes UP.Shim under the front base POI goes Down

Check and make sure is no sticky paper in one of the rings and not in the other sometimes there is paper there.a Black sticky thats help to stop marring the scope.

09-03-2007, 08:46 PM	#1
B&E Detective Member Join Date: Sep 2007 Location: virginia Posts: 2	request for formula for scope shim Does anyone have a formula as to shim thickness to raise my front scope mount approximately 4 inches higher from 75 to 100 yards. At the top of my scope travel I'm still 4 inches low at 100 yards. I placed a coke can sliver in the front mount which is .005" thickness, but I'd like a formula to figure out correct thickness to correct this problem. -->

09-03-2007, 09:56 PM	#2
strayshot V.I.P. Member Join Date: Aug 2005 Location: upper midwest Posts: 359	Re: request for formula for scope shim I don't know the shim thickness and there are more experienced folks on here then me but, If you're saying the bullet hole is 4" lower than the crosshairs, than you need to move the scope down...unless I'm not understanding here. __________________

09-04-2007, 06:44 AM	#3
Don Buckbee Senior Member Join Date: May 2004 Location: Grayling, MI Posts: 712	Re: request for formula for scope shim Burris Signature rings, the ones with the adjustable inserts, are a far better solution to getting a scope zeroed than shimming. Shimming one ring higher puts a stress on the scope tube, which is not good. Don __________________ Don JPFO NRA Benefactor Member NSSA Life Member

09-04-2007, 08:50 AM	#4
LDBennett Advanced Senior Member Join Date: Dec 2003 Location: Hesperia, CA Posts: 5,715	Re: request for formula for scope shim Its a ratio thing. You are comparing how far you want to move the bullet on the target with its shooting range and how much shimming required over the length between the mounting points on the gun. (bullet errror in inches) x (receiver mounting distance in inches) divided by (range in INCHES) = shim thickness in inches Note all measurement are in the same measuring units, inches in this case. If it is shooting low now then you want to elevate the rear of the scope to make the scope look where the bullet is going. You'll need more than the calculated shim because you don't want to end up eith the scope at the extreme end of its adjustment as you now have it. Doubling the above calculation results might get you closer to the ideal. If you have to use too big a shim you are indeed going to stress the scope tube. The Burris plastic insert mounts are the real answer, not shims, if the required shim stock is too much. When you use thick shim stock you move the stress of the recoil to the screws as the mounts are not down on the receiver and the friction of that juncture is not assisting resisting the recoil. The screws see more of the recoil forces and may snap off if the shimming is too extreme. The Burris mounts eliminate that problem and look neater too. Run through the above calculation and review the Burris application information to see what set of offsets you need. I'm not familiar with how they spec them but I would think the instructions that come with them would be helpfull. You can use a bore sighting laser tool to help in all of this. LDBennett

09-04-2007, 08:33 PM	#5
B&E Detective Member Join Date: Sep 2007 Location: virginia Posts: 2	Re: request for formula for scope shim I appreciate all the answers to this problem. I'm looking at and ordering a set of Burris Signature rings this evening. Thanks again.

10-09-2007, 08:00 PM	#6
ohsix Member Join Date: Mar 2005 Location: VA Posts: 7	Re: request for formula for scope shim Distance in Yards Shim Thickness Point of impact in inches 25 .001 .250 50 .001 .500 100 .001 1.0 200 .001 2.0 25 .005 1.25 50 .005 2.5 100 .005 5.0 200 .005 10.0 When the rings are 3.25 inches spacing. I would try a .004 shim or a .006 shim under the rear base and see if U can adjust the scope.When U raise the rear base the POI goes UP.Shim under the front base POI goes Down Check and make sure is no sticky paper in one of the rings and not in the other sometimes there is paper there.a Black sticky thats help to stop marring the scope. Last edited by ohsix; 10-09-2007 at 08:12 PM..