Rifle Scopes Height of Scope over bore

pschwenke

Private
Minuteman
Jun 6, 2010
12
0
52
If you are running a 20 MOA base, where do you measure the center of your scope for Ballistic programs? Seems like the canted scope mount could add in error? What's the proper method?

I have a 5R milspec with Wotac Gen5 4-14, Burris xtr low mount rings, and EGW 20MOA mount.

Paul
 

remaction

Sergeant
Full Member
Minuteman
Oct 22, 2008
589
171
61
Plain, WI
Re: Height of Scope over bore

Paul,

First off, if I'm way off in this someone let Paul and I know that I'm full of sh*t.

Interesting question for several reasons but I'd say you'd be splitting hairs in trying to get the right theoretical answer. I'd say just use the height to the center of the scope at the rear base as I believe in a second focal plane scope the reticle is nearer the eyepiece. I'm not 100% positive on this, as it's only based on information I've seen. You'll just have to run the calc's then verify in the field.

It's an interesting question because in a first focal plane scope, I believe the reticle is near the turrets and actually moves up and down based on where the erector tube is within the adjustment range and I've not read where anyone has been concerned about where their reticle is in relation to the center of the scope tube.
 

Aries64

Deep Behind Enemy Lines in IsWokestan!
Full Member
Minuteman
Mar 12, 2007
2,560
310
Los Angeles, California
Re: Height of Scope over bore

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: PSchwenke</div><div class="ubbcode-body">If you are running a 20 MOA base, where do you measure the center of your scope for Ballistic programs? Seems like the canted scope mount could add in error? What's the proper method?

I have a 5R milspec with Wotac Gen5 4-14, Burris xtr low mount rings, and EGW 20MOA mount.

Paul </div></div>
I measure from the center of the action's pressure relief hole to the center of the tube, and prefer to use the center of the Parallax or Windage turret as the tube center. Because the turrets are usually located pretty close to the midpoint on the tube, measuring at the turret center should help to minimize height measurement error between FFP amd SFP reticles. And if the turrets happen to fall at or close to the midpoint of the base the height differential between the front and rear will also be minimized.

Also, if you use the <span style="font-weight: bold"><span style="font-style: italic">JBM Small Arms Ballistics Trajectory calculator</span></span> you can enter the a value to compensate for the cant in a base under <span style="font-style: italic">"Line Of Sight Angle"</span>. The <span style="font-style: italic">"Line Of Sight Angle"</span> is the relationship between the barrel's plane and the scope's LOS. Since there are sixty (60) Minutes in one (1) Degree, the closest approximate corresponding value for a 20 MOA base is .33 MOA (1/3 of one Degree).

Make sure you enter the value under <span style="font-style: italic">"Line Of Sight Angle"</span>, <span style="font-weight: bold"><span style="text-decoration: underline">NOT</span></span> <span style="font-style: italic">"Cant Angle"</span>. The <span style="font-style: italic">"Cant Angle"</span> setting refers to the angle of the barrel <span style="font-style: italic">in relation to the target</span>.

You'll always have to confirm the accuracy of calculations of any ballistics program by shooting, but my measurement method has always worked for me when using JBM. I've used 20 MOA bases as well as 40 MOA bases and I've never been off more than a single click of the adjustments.


Keith
 

pschwenke

Private
Minuteman
Jun 6, 2010
12
0
52
Re: Height of Scope over bore

Thank you all for the responses. It make sense to use the center of the tube. I like knowing all factors going into the ballistics calculations. Most people in my area wouldn't give a rip about this error.

Paul
 

Aries64

Deep Behind Enemy Lines in IsWokestan!
Full Member
Minuteman
Mar 12, 2007
2,560
310
Los Angeles, California
Re: Height of Scope over bore

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: PSchwenke</div><div class="ubbcode-body">Thank you all for the responses. It make sense to use the center of the tube. I like knowing all factors going into the ballistics calculations. Most people in my area wouldn't give a rip about this error.

Paul </div></div>
I took it as a given so I didn't mention it before, but for the record - if you aren't already using them, I highly recommend that you pick up a Kestrel Pocket Weather Meter and a decent chrono. You need accurate muzzle velocity and environmental data as well as accurate BC data to get the best results out of any ballistics program.

I may get blasted here, but stay away from the low end of the scale such as the "Shooting Chrony". While they are inexpensive and widely available they are cheaply made, inaccurate and consistent POS'. On the low end of the chronos' I'd use the CED Millenium 2, with the high end of the consumer-friendly units being the Oehler Model 35/35P and the PVM 08.

The more accurate the data you input into a balllistics program the more accurate the data you get from said ballistics program will be. (<span style="font-style: italic">"Garbage in - garbage out"</span>).


Keith
 

Rob01

Super Mod
Staff member
Moderator
Full Member
Minuteman
Jul 9, 2001
12,293
4,817
NC
Re: Height of Scope over bore

Been covered many times. Here's a thread
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1632313

From a link in the above link to another thread on it
P6120517.jpg


or this
P6120518.jpg


Both work just fine for adding height to ballistic software. No need for complicated formulas.
 

pcschwenke

Sergeant
Full Member
Minuteman
Apr 10, 2008
226
0
Northern Michigan
Re: Height of Scope over bore

I currently have a Pact MKIV timer/chronograph. It seems to work well in both modes and has a ballistic program as well. Not up to today's standards though. When thumbing through this program last week, it wanted to know the scope height from the bore. Never really thought about it before this. The angled base through me off.

I have been looking at the Pocket weather meters. Wind is very hard to judge correctly.

Paul