# Line of site 1.5" above center of rifle barrel?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by jmden, Jun 8, 2004.

1. ### jmdenWell-Known Member

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Hi,

I've noticed that in my computer ballistics program (Sierra), one of the data points asked for to predict bullet travel in relation to how your gun is set up is the height of the scope above the barrel, or line of site to center of barrel distance. (I'm probably butchering this description...sorry.) Changing this number by a couple of tenths of an inch seems to change the calculations of the computer program, sometimes what I would consider a significant amount. So, I wonder if the "standard 1.5 inches" often used might be causing practical discrepancies between computer program and data from the range/field. If the computer program is accurate in this respect, and because of it this measurement needs to be precise, my question is: How do you reliably, precisely measure this distance?

Thank you,

Jon Denham

2. ### Dave KingWell-Known Member

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Measure, or know the tube size on your scope.

Measure the barrel thickness with a caliper.

Measure the distance from the barrel to the scope tube at the point you just measured the barrel thickness.

Add 1/2 the scope tube diameter, 1/2 the barrel diameter and the entire barrel to scope tube distance.

For example... a 1" scope, a 1.25 inch barrel and a .4 inch barrel to scope value. .5 + .625 + .6 = 1.725"

This distance is used to determine the necessary "up" angle on the barrel to make the bullet trajectory cross the line-of-sight at the zero distance. As I understand it this is how it works. For 100 yards we'll say a bullet drops 2.5 inches from the centerline of the bore once fired. If the scope is 1.5 inches above the bore centerline an additional 1.5 inches of "up" must be introduced to get the bullet to cross the line-of-sight at the 100 yard zero distance (so a total of 4" of "up" is required, not the somple 2.5").

3. ### 4ked HornWell-Known Member

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Jun 13, 2007
Dave hit it on the head but it might be easier to measure the outside diameter of the objective lens bell, and the gap between the bell and the barrel, since they are away from all the stuff at the action. It is sometimes hard to get calipers in around that stuff.

Same formula would apply though:

1/2 barrel diameter
+
1/2 objective bell diameter
+
Gap between bbl and bell
=
Distance from bore to line of sight.

Some might say that the closer your scope is to the bore the better off you are but if you are dialing in your range it dosen't matter too much. But that is another topic all together.

4. ### jmdenWell-Known Member

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Thank you Dave and 4ked Horn. I did this today and found that I was within a few thousandths of what I had roughly estimated with a ruler--approximately 1.706.

I think I could've figured that out that method myself if I'd put a little more thought into it.

Thanks,

Jon Denham

[ 06-09-2004: Message edited by: Jon Denham ]