Rifle Scopes Determining scope height above bore for ballistics

#2
Re: Determining scope height above bore for ballistics

I <span style="font-weight: bold">believe</span> it is measured C2C from the leading edge of the scope.

That measurement has always jived with my ballistic software ( Exbal ) I use this for workin up loads and figuring actual velocities only though.
 
#3
Re: Determining scope height above bore for ballistics

Tim,

I actually measured mine. I took half the barrel diameter, at the point where the objective ends over the barrel. Then added the distance between the top of the barrel, and the bottom of the objective bell. The I took half of the OD of the outside of the objective end of the scope.

So for my rifle, Rem 308 LTR, w/ IOR 3-18x42MM, TPS Ex LOW rings; it worked like this:

Barrel .935/2= .4675"
Scope HT over barrel = .451"
Objective OD 1.910/2 = .955

Total: 1.8735" Height above centerline bore.

It took less time to measure than to write this.

Bob
 
#4
Re: Determining scope height above bore for ballistics

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BobinNC</div><div class="ubbcode-body">Tim,

I actually measured mine. I took half the barrel diameter, at the point where the objective ends over the barrel. Then added the distance between the top of the barrel, and the bottom of the objective bell. The I took half of the OD of the outside of the objective end of the scope.

So for my rifle, Rem 308 LTR, w/ IOR 3-18x42MM, TPS Ex LOW rings; it worked like this:

Barrel .935/2= .4675"
Scope HT over barrel = .451"
Objective OD 1.910/2 = .955

Total: 1.8735" Height above centerline bore.

It took less time to measure than to write this.

Bob </div></div>

That ^^^ is the technical version of what I was tryin to say. Well put BobinNC.
 
#5
Re: Determining scope height above bore for ballistics

This is how I do it... measure the bolt diameter, take one half of that value and then measure the scopes tube diameter and take half of that and the measure the distance from the top of the bolt to the bottom of the scope. I add all of these values together and it tells me the distance between the rifles bore axis and the scopes axis..which is scope height
smile.gif
. Hope this helps.

Kyle
 
#6
Re: Determining scope height above bore for ballistics

All of that is way too anal.

Take a ruler. Place the bottom of the rule on the approximate center of the bolt.

Measure from there to the center of the scope tube.

Done.

It's not a critical measurement. If you don't believe that, set up a ballistic program with what you measure. Change the value of that measurement from what you measure by 1/4 inch, and notice that it makes little difference downrange.
 
#7
Re: Determining scope height above bore for ballistics

Lindy,

I did just that, and your right, it did not change things much down range, a tenth of a mil here and there. However, running JBM with mechanical offset calculation on ranges 5 to 95 yds in 5 yd increments, I did see major MIL difference's.

Yes, overall we are talking small potatoes, but when your trying to hit a 1/2" dot at a recent competition, at a target 5 yds. away, and your shooting down at it from a tractor trailer truck roof, misses by inches seem to abound. Some shooters missed the entire 8 1/2 by 11 target altogether.

Sobering experience. What are your thoughts on height above bore and the mechanical offset problem at spitball range.....

Thanks,

Bob

 
#8
Re: Determining scope height above bore for ballistics

Practice.
laugh.gif


You can use a ballistic program to get some idea of what it takes to hit those shots. However, practically speaking, when the angle down varies, it gets pretty tricky.

Although a half-inch dot is pretty easy. When we did the 11 foot 1 inch shot at a Rifles Only match, the target was a quarter-inch dot.
laugh.gif
 
#9
Re: Determining scope height above bore for ballistics

Personally, I prefer to have my data as close to actually be spot on as possible. This enables me to rule out one more factor that may cause my shot to be off. I agree that it is not that crucial to have the sight height dead on, however, for no more time than it takes...why not?
smile.gif
 
#13
Re: Determining scope height above bore for ballistics

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: KyleFreemason</div><div class="ubbcode-body">This is how I do it... measure the bolt diameter, take one half of that value and then measure the scopes tube diameter and take half of that and the measure the distance from the top of the bolt to the bottom of the scope. I add all of these values together and it tells me the distance between the rifles bore axis and the scopes axis..which is scope height
smile.gif
. Hope this helps.

Kyle

This worked perfectly and allowed me to take my measurements at about the same point where I think the reticle is inside the scope. Thanks to all for the input!

</div></div>
 
#14
Re: Determining scope height above bore for ballistics

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">All of that is way too anal.

<span style="font-weight: bold">Take a ruler. Place the bottom of the rule on the approximate center of the bolt.

Measure from there to the center of the scope tube.

Done.</span>

It's not a critical measurement. If you don't believe that, set up a ballistic program with what you measure. Change the value of that measurement from what you measure by 1/4 inch, and notice that it makes little difference downrange.
</div></div>

DITTO

If you measure carefully the way Lindy indicated, the error is 1/8" or even less, and no one is going to miss the target at any range due to this.

The mechanical offset error is corrected by a simple formula, and has nothing to do with the particular rifle/load ballistics.

You don't need a program to know the error induced, if you have a 1/8" error in scope height, this is about 1/8" error at the muzzle.

With a 100 yds baseline zero, this is 0.5*1/8" = 0.0625" error at 50 yds; zero at 100; and only 9*1/8" = 1.125" error at 1000 yds, with any load.
 
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