Optic measurement

[sc1911cwp]

I see in some Ballistics programs that one element that can be entered is the height above barrel. I think this measurement is from the center of the bore to the center of your optic. Is there an accurate way to do this besides just using a tape. Does it matter?

[Hotolds442]

Going from 1.5" above bore centerline, to 2" above results in a calculated 2" difference in drop at 1000 yards in one of my rifles.

[SKami]

I see in some Ballistics programs that one element that can be entered is the height above barrel. I think this measurement is from the center of the bore to the center of your optic. Is there an accurate way to do this besides just using a tape. Does it matter?
I think it does matter for a number of reasons, one is available scope adjustment, and two is the angle of the bore center relative to true horizontal. This subject interests me greatly so I will tag along, but it seems to have spawned a whole industry based on POI changes relative to angle of shot-i.e. the newest range finders that take that angle in to account.

[yobuck]

I think it does matter for a number of reasons, one is available scope adjustment, and two is the angle of the bore center relative to true horizontal. This subject interests me greatly so I will tag along, but it seems to have spawned a whole industry based on POI changes relative to angle of shot-i.e. the newest range finders that take that angle in to account.

Scope hgt above the bore will have an affect on elevation adjustment. A measurment from the gas port to center of the scope is close enough for most use.
In other words the higher the scope above bore the less clicks required at a longer distance. It has no affect on the total number of clicks the scope will produce however as thats internal.
Shooting uphill/downhill angles is a different subject unrelated to the scope other than the correction needed to compensate for it.

[SKami]

Does the POI expand exponentially at greater distances, or does it stay the same? I mean, if one scope is 1.5 inches above the barrel, and one is 2.5 inches above the barrel-and both are parallel to each other, and horizontal will the poi difference be 1 inch always? Also, I get the shooting uphill/downhill thing, but a scope mounted higher requires more upward angle to the barrel for line of sight to cross barrel centerline( bullet path)-that would be a steeper angle and should change trajectory to the extent that the bullet now has a slight vertical component, I know I'm confusing everyone, and over thinking things, but just wondering? I think I understand that less adjustment is required at longer ranges, because you are starting out with a steeper angle to begin with, is that correct?

[SK13R]

I usually just measure from the split in the scope rings to the gas port if I can. I don't get to crazy. Take a look at the data below for one of my loads. Only thing I changed was scope height. I changed it by .5" The difference is pretty small for such a large change in scope height. Only 4.5" at 1000 yards.

1.2" Scope height (not actual height but just for illustration purposes)
Range Drop Drop
(yd) (in) (mil)
0 -1.2 ***
100 -0.0 -0.0
200 -3.7 -0.5
300 -13.6 -1.3
400 -31.3 -2.2
500 -58.7 -3.3
600 -98.7 -4.6
700 -154.8 -6.1
800 -231.3 -8.0
900 -333.3 -10.3
1000 -465.5 -12.9

1.7" Scope Height
Range Drop Drop
(yd) (in) (mil)
0 -1.7 ***
100 -0.0 -0.0
200 -3.2 -0.4
300 -12.6 -1.2
400 -29.8 -2.1
500 -56.7 -3.2
600 -96.2 -4.5
700 -151.8 -6.0
800 -227.8 -7.9
900 -329.3 -10.2
1000 -461.0 -12.8

[chukarmandoo]

That 4.5" seems very small and you could shine it. Then you could shine the Coriolis and that would be very small. Then you could shine Spin drift because it would seem small.Then add shooter error and pretty soon you have nothing! With all the programs we have available today it is very wise to use all the numbers you can to the best of our ability. It just makes us that much better. Thats the reason people write books on this stuff. But then there is always the Hal-o-Mary!

[SK13R]

Oh I agree. I measure to the tenth for that variable. I was just trying to illustrate what I thought was a huge difference in scope height created a small difference at that distance. So the more accurate you measure it the better off you are. But if your are off by a tenth of a inch in your measuring you are talking less than a inch at a thousand.
But hey, sometimes the guesstamation shot is fun to try too. ;)

[sc1911cwp]

So, is there a way to easily do this? I accept that it makes a difference, however these are my concerns. I can account for half of the 30mm tube. This is the distance from the rail base to the center of the scope ring. How do I account for the center of the bore to the rail base? How can I easily measure that on something that has a contour? I'm new so I pled the 5th.

[SK13R]

Measure to the top of your bolt. Then add half the diameter of the bolt should get you real close.

[chukarmandoo]

I measure the objective bell and divide that into. Then measure from bottom of bell to rifle barrel. Then measure rifle barrel and divide into. Add the three numbers you come up with and that is your scope height. I measure at the front of the scope.

[DrThunder88]

Does the rifle have a round receiver? I'd say measure the diameter at the front receiver bridge and divide the measurement in half. Measure the height of the rail above the receiver. Then, either use the factory measurements for the rings or measure the distance between the top of the rail and the bottom of the scope tube. Finally, add half the diameter of the scope tube.

This assumes you have dial calipers. If not, get some! Even the cheap ones are quite useful.

[yobuck]

so whats wrong with the simple method suggested by SK13R ? That being gas port to center of scope or scope ring split.

[chukarmandoo]

Not a thing! I just like to work the numbers from the Objective lens with a canted rail. Might not make a difference though.

[sc1911cwp]

TNX for all the suggestions.