Thread: Changing ballistic info to match "real" drop.

After having done some set up with a new load for my 7mmRM and entering all of the relevant information into 2 different ballistic calculators (which both came up with the same trajectory data) I have found a difference in the actual drop (zeroed at 109 yards and tested at 440 yards) and the correction provided by the calculators.
It is shooting 3" high at 440 yards with the recommended dial up of 6.7 MOA.
I have only used one chronograph to check MV and I am relying on Noslers BC data from their web site.
My question is, what is the best way to change the calculation to match the actual drop?
Change the MV? Change the BC? or is there something I am missing?

What ballistic programme are you using mate? Assuming all the atmospherics entered are correct some apps such as Applied Ballistics will allow you to true your data. I.E enter what your actual elevation was at a number of given ranges. The app will then provide a "trued" ballistic table for that environment. My chrony isnt that reliable so use that for ballpark and then fine tune using the above method. Hope that makes sense?

try changing velocity by say 25fps at a time and see what it does.......

Originally Posted by Link

What ballistic programme are you using mate? Assuming all the atmospherics entered are correct some apps such as Applied Ballistics will allow you to true your data. I.E enter what your actual elevation was at a number of given ranges. The app will then provide a "trued" ballistic table for that environment. My chrony isnt that reliable so use that for ballpark and then fine tune using the above method. Hope that makes sense?

Thanks for that. I have all the correct atmospheric data for when I sighted it in and current conditions so that shouldn't be an issue.
When you say the app "trues" the data, what does it adjust to true it? This is what I am getting at.

Originally Posted by Micky Duck

try changing velocity by say 25fps at a time and see what it does.......

+1 I do the same, just tweak velocity til it matches where the group landed on target

Originally Posted by Micky Duck

try changing velocity by say 25fps at a time and see what it does.......

Yeah. Just had a play with that. It's about 100fps difference. Seems a big difference but as I said, only used one chronograph.
The "new" velocity is not unrealistic however and actually more in keeping with what I expected when I first developed the load.

Brian Litz has an excellent chapter on this topic in one of his books - well worth reading.

He basically says change the variable that you have the least faith in.

There is a fair amount of documented research for most of the modern projectiles, and you should be able to get a validated BC (G7) for them.
DO NOT trust what the manufacturer tells you their BC value is, it is marketing. Let me know which projectile you are using and I can look it up in Litz's book.

Chronographs, unless you are using a LabRadar, are not exactly precise instruments, so muzzle velocity would more than likely be the variable I change first.

The other thing to confirm is that your turrets are calibrated correctly. Shoot a "tall target" and get a good 100yd zero, then dial up 10 or 20 minutes, and measure the shot fall.

Cheers @ebf.
Projectile is a .284 150gn Nosler BT. Nosler recon .493

G7 0.229 G1 0.447

Use G7, it should be a better match for that shape projectile

That's from 2nd Edition of Applied Ballistics, someone else may have a later version to confirm recent values.

Cheers. I will use that and see where that puts the velocity.

Unfortunately that just makes it worse. On the bright side my MV is climbing all the time.

Out of interest @ebf, do you have BC info on the .284 160gn Seirra TMK?

Nope sorry, TMK's came out after my book was published.

Hey @Shearer are you using a mill dot scope by any chance? If so have you calibrated it to find out if it's correct.
I.e. The mills are correct quite a few are round about someone will be along to say how. Just a thought.

Weird, on JBM Ballistics they have a 155gr TMK and a 168 TMK ?

If you select the (Litz) projectile from their library and look on the result page, it lists the relevant BC