Range Report Help me check my math please Re: Ballistics/JBM

[TheGerman]

Was seeing if my calculations and the way I was looking at this were correct. I was planning on shooting out to 1k with a 175gr SMK in 308Win, and eventually pushing things out to 1500 with the 208 AMAX. In order to be able to do this with my current rig, I wanted to see if I had enough MOA to get out there as I currently have a 0 MOA base.

I went to JBM, and put in the variables for a 175gr (.249BC) SMK @ 2650FPS. Altitude where I am at is roughly 2860 feet and I averaged I would be shooting in about 75 degrees.

The data JBM gave me out to 1k was this:

Range Drop Drop Windage Windage Velocity Mach Energy Time Lead Lead
(yd) (in) (MOA) (in) (MOA) (ft/s) (none) (ft•lbs) (s) (in) (MOA)
1000 -365.3 -34.9 0.0 0.0 1290.9 1.139 647.4 1.620 0.0 0.0

So by going with this, I would need 35 MOA in the scope to get out to that range correct?

The optic I'm using is a Leupold Mark 4 3.5-10 M2. The documentation on it says that it has a 120 MOA range in elevation; dividing that by half I am still at roughly 60 MOA positive elevation adjustment, 25MOA over what I would need at 1k yards. Or am I missing something?

Past 1k yards, I'd switch to the 208 AMAX (.648 BC) at 2650fps. JBM tells me that we'd need only 42.2 MOA to get there.

Range Drop Drop Windage Windage Velocity Mach Energy Time Lead Lead
(yd) (in) (MOA) (in) (MOA) (ft/s) (none) (ft•lbs) (s) (in) (MOA)
1500 -662.9 -42.2 0.0 0.0 1809.0 1.596 1511.2 2.056 0.0 0.0


Again, am I missing something, or is the way I am looking at this and are the figures I am using correct?

Thanks



Edit: Corrected 1500 yard data for 208 Amax ballistics.

 

[CoryT]

Help me check my math please Re: Ballistics/JBM

The estimated come ups are fine. The 3.5-10 Mk4 has 90 MOA total though, the fixed 16x has 140.

That in and of itself does not answer how much travel you actually have once the scope is mounted and zeroed. Depending on bore height, ring and base alignment, action to barrel alignment, etc., you could end up with only 28MOA up after zero at 100. Most .308 systems will be best served with a 20-30 MOA slope in the base.

 

[MitchAlsup]

Help me check my math please Re: Ballistics/JBM

Your math is fine (with the 90 MoA limt CoryT added).

If you mount your optic on a 20 Moa or 30 MoA base, you can get more vertical adjustment range. Somewhat unnecessary for a 308 with a 90 MoA scope, however.

 

[RL15]

Help me check my math please Re: Ballistics/JBM

When your scope is in its mechanical center it has its max windage availible.
The further you adust the elevation the less windage availible.

when you add your 42-43 MOA on your scope how much windage do you have availible?