Finding Scope Center Height to Center Line of the Bore for Ballistic Programs
One of the questions the Sinclair technical staff receives on a regular basis is how one can find the scope center height to the center line of the bore for their rifle. This measurement is one of the variables used by most ballistic programs. The following is the method I use to determine this measurement, it is not exact, but its close enough for trajectory calculations for most rifles and ranges:
1.) Measure the bolt diameter and divide it in half. Example 0.700”/ 2 = 0.350”
2.) Measure the scope tube diameter and divide it in half. 1.000” / 2 = 0.500” (for 30mm tubes use 1.181”)
3.) Measure the distance from the top of the bolt in the rifle to the bottom of the scope on the rifle. Example 0.750”
4.) Add the numbers found in the formulas in steps 1-3 (0.350” + 0.500” + 0.750”= 1.600”)
Most load manuals and scope companies use 1.500” scope heights above the bore for their trajectory tables, since it is a very common measurement. Also most ballistic programs have 1.500” as the default measurement. If you are shooting longer distances such as 500 yards or further, the above formula should help you determine more accurate trajectories.
I personally try to keep the scope as close to the center of the bore as possible. Ideally, I like to have a 0.030” to 0.050” clearance between the bottom of the scope’s front bell to the barrel. I try different base and ring height combinations to achieve this clearance. For example on most of my Remington’s I use Ken Farrell bases with Leupold rings. I use their PRW rings for hunting and varmint rifles and their QRW rings for target rifles allowing me to remove my scope so I can also shoot with iron sights. If you have questions on how to determine the appropriate ring/base height, please call Sinclair International and ask one of our technicians for help.
Ron Dague
Sinclair International Reloading Instructor/ Tech and Sales Rep
800-717-8211
rond@sinclairintl.com
October 28th, 2010 at 10:23 am
That’s so simple it’s brilliant. Thanks.
November 19th, 2010 at 3:04 pm
Could you please define “bottom of the scope”? Does that mean the bottom of the tube or the eye piece (which I imagine will vary from manufacturer)?
November 19th, 2010 at 4:27 pm
Heck, even I can do that !!!
November 19th, 2010 at 6:18 pm
Ha! One of those things I always guesstimate at…this is too easy.
Thank you!
November 20th, 2010 at 6:59 am
Formula much more precise then the generic “1.5″…it may not make a big difference but WHO knows?
Thanks.
November 21st, 2010 at 10:29 am
Ron, How would you make this determination when using say a 20 MOA ramped base where scope tube is not parallel with barrel bore to start?
November 22nd, 2010 at 12:08 am
I agree, I like low mounted scopes also. I use the Burris Signature rings whenever possible.
November 22nd, 2010 at 6:35 pm
Lay the rifle down on any surface that has parallel lines, such as a wood floor. Align the rifle until one of the parallel lines passes exactly down the center of the scope as you view it from directly above the scope. Lay a piece of paper down so that the edge of the paper sits exactly on that line, and the paper extends under the muzzle of the rifle. Using a very sharp pencil, look straight down on the muzzle and make a mark on the paper exactly in the middle of the muzzle. Measure the distance from the mark to the edge of the paper.
Be sure to be very picky about how you line up the scope, how you line up the pencil with the muzzle, and how you pick the straight line on the surface.
December 7th, 2010 at 11:05 am
Dear Joe,
The main question that we get ask is about center of scope to center of the bore. This is for a more precise measurement for scope height to center of bore, this is not an exact measurement. The 20 moa shouldn’t make take a difference. This is just for reference purpose. If you look in reloading manuals they use an 1.5 above center of bore. I think knowing a scope height is 1.6 over the bore is good info for long range shooting. Short range I don’t think it is as important at say 200 yards or less.
If you have questions please call 800-717-8211
Thank you,
Ron
December 7th, 2010 at 11:05 am
Dear Jesse,
The bottom of the scope I am referring to is 1 inch scope tube and on 30mm. This has no reference to scope bell on either end of scope.
If you have any questions please call 800-717-8211
Thank you,
Ron