Advanced Marksmanship Calculating sight height with canted base?

[High Binder]

Ballistic software (Sierra and the like) all ask for a sight height number referring to what I believe is the center-line of the scope above the bores center-line. So how does one calculate this number given a base with say a 20MOA for example cant built in?

 

[Lowlight]

Re: Calculating sight height with canted base?

You don't you just give it the overall height, it's not something you need to figure.

Simply go center of the scope tube to center of the bore and be roughly within 1/8th of an inch and you are ok.

Don't get wrapped around the axle.

 

[High Binder]

Re: Calculating sight height with canted base?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lowlight</div><div class="ubbcode-body">
Don't get wrapped around the axle. </div></div>

Rodger that, thanks for the info. I was over-thinking it.

 

[Rob01]

Re: Calculating sight height with canted base?

Measure from the top of the stock or the hole, which is close to center of bore, up to center of windage knob.

 

[TallShot]

Re: Calculating sight height with canted base?

This is why I love being here. Good info, and something I didn't even think about...

 

[JimGnitecki]

Re: Calculating sight height with canted base?

The trick is remembering to factor in that 10 or 20 MOA that is "pre-built-in" when using the results of the ballistics software!

i.e., if the software says you need 35 MOA, rmember that you already have 15 dialed in via the 20 MOA rail or mount!

Jim G

 

[Biggerhammer50]

Re: Calculating sight height with canted base?

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.

 

[BigJohn141]

Re: Calculating sight height with canted base?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JimGnitecki</div><div class="ubbcode-body">The trick is remembering to factor in that 10 or 20 MOA that is "pre-built-in" when using the results of the ballistics software!

i.e., if the software says you need 35 MOA, rmember that you already have 15 dialed in via the 20 MOA rail or mount!

Jim G </div></div>

Ok I could very well be wrong on this and please correct me if I am. You shouldn't see and change and any of your ballistic program's numbers. All the 20 MOA base does is allow you to dial down you scope to get more usable elevation out of it. If your program says you need 35 MOA from zero for X distance then you still need to dial 35 MOA. Correct? Sorry to hijack the thread.

 

[EXTREMEPREJUDICE]

Re: Calculating sight height with canted base?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BigJohn141</div><div class="ubbcode-body">Ok I could very well be wrong on this and please correct me if I am. You shouldn't see and change and any of your ballistic program's numbers. All the 20 MOA base does is allow you to dial down you scope to get more usable elevation out of it. If your program says you need 35 MOA from zero for X distance then you still need to dial 35 MOA. Correct? Sorry to hijack the thread. </div></div>

Absolutely correct......

 

[Biggerhammer50]

Re: Calculating sight height with canted base?

The example I posted earlier will account for the additional 20,30 or what ever MOA base you choose by the measurement from the top of the bolt to the bottom of the scope. As Lowlight put it "don't get wrapped around the axel" Its not that technical.

 

[Greg Langelius *]

Re: Calculating sight height with canted base?

Half the bolt diameter, plus half the scope tube diameter, plus the height of the gap Between them. If there's slope, measuring the gap at halfway between the rings is close enough.

As LL says, the tolerances needed here are not microscopic.

The reasoning behind that is simply that even if you got it right at the microscopic level, outside factors, like uncontrollable environmental factors affecting trajectories, will pretty much always negate the finer precision in the overall calculations. If this were not so, all groups would be one bullets hole's width in diameter.

And I think we all know that's never gonna happen for folks like us.

Calculation may warm one's heart, but reality is still not one's best friend.

Greg

 

[gafferq2xl]

Re: Calculating sight height with canted base?

Nice one Rob. That made it stupid easy. Wish I'd known that years ago.

 

[Country]

Re: Calculating sight height with canted base?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Biggerhammer50</div><div class="ubbcode-body">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. </div></div>
+1 That's the way I do it also.

 

[Sterling Shooter]

Re: Calculating sight height with canted base?

O.K. you've followed the instructions and you end up with a sight that is too high or too low when a proper stockweld is applied. Think it all out folks, the idea is to end up with a scope that not only clears the bolt, bridge and barrel but will allow for a muscularly relaxed stockweld where the shooter's eyeball is aligned with the eyepiece when resting the full weight of the head on the comb. Usually, both proper mount height and comb height must be considered to better assure parallax reduction.

 

[Ryken93]

Re: Calculating sight height with canted base?

The trick is remembering to factor in that 10 or 20 MOA that is "pre-built-in" when using the results of the ballistics software!

i.e., if the software says you need 35 MOA, rmember that you already have 15 dialed in via the 20 MOA rail or mount!

Jim G

This is not true… your scopes internal travel offsets the rail and you zero your scope crosshairs at POI.
The only way your theory is true is if you have like a 100+ moa rail, and your internal travel can’t match it so your POI is higher than your scopes crosshairs. And at that point you wouldn’t do an MOA add/deduct, you’d actually calculate the offset into the equation and get the correct moa.

 

[Aftermath]

This is not true… your scopes internal travel offsets the rail and you zero your scope crosshairs at POI.
The only way your theory is true is if you have like a 100+ moa rail, and your internal travel can’t match it so your POI is higher than your scopes crosshairs. And at that point you wouldn’t do an MOA add/deduct, you’d actually calculate the offset into the equation and get the correct moa.

11 years ago....