T2013 STAGE III Ballistics Calculation Question

[3catcircus]

So,

I'm going through and statting up some new weapons and ammo. Stuff like the S&W M&P 10 or the LSA TX15. These are using 6.5 Creedmoor or 6 ARC.

While I could just say that these are equivalent to the 6.5 Grendel, my intellectual curiosity demands that I figure out Stage III ballistics values for them.

Anyone know how Stage III ballistics are calculated? The math doesn't scare me, I just need to understand how the formulas vary in each range band.

[3catcircus]

So I put a little more thought into this (and applied liberal use of Hornady's online ballistics calculator)...

Since the Master Ammo Table has .17 HMR but without any Stage III ballistics, I figured I'd start out with that as a test case.

Knowing that the range bands are:
Personal (arms length) - I assumed 1 meter
Gunfighting (7m)
CQB (7-25m)
Tight (25-100m)
Medium (100-200m)
Open (200-400m)
Sniping (400-800m)
Extreme (800-1600m)

I plugged in the values for the Hornady .17 HMR (17 gr V-Max Varmint Express), and chose max range 1600 yards, interval 25 yards. It generated velocities and kinetic energies every 25 yards (close enough to meters for me).

The results I got were then plugged into the Reflex ballistics calculator, giving me (assuming the first penetration value rather than the second):

P 4/x2
GF 4/x2
CQB 4/x2
T 3/x3
M 3/x3
O 2/Nil
S 2/Nil
E 2/Nil

The question becomes - do I select the *start* of each range band, the *middle* of each range band, or the *end* of each range band?

The reason I ask is there are some significant differences in V and KE across the range band. For example, for the Tight range band, V = 2377 fps and KE = 213 ft-lbs at 25m, but drops off to 1902 fps and 137 ft-lbs at 100m.

This results in values of 4/x2 if I use the start of the band (25m), 3/x3 if I use the end of the band (100m) and 4/x3 if I use mid-band (I've been lazy and just assumed a linear decrease in velocity and kinetic energy across the distance the bullet traveled).

Additionally, do the ranges even make sense (knowing that .17 HMR is intended for small game/varmint hunting). The design guidelines specify Open/Extreme as optimum/maximum ranges for hunting rifles, but I'm thinking a rifle in this caliber is probably Gunfighting/Tight or Gunfighting/Medium.

**EDIT**

I'm thinking optimum and Max range for a rifle firing .17HMR is going to be M/O. Plotting velocity and energy vs range, the knee in the curves is around 250 - 300m. Anecdotally, hunters are claiming good shots too 200 yds consistently.

[3catcircus]

Here's my first attempt at Stage III Ballistics for a few new calibers of ammo.

Thoughts?

[Tegyrius]

Originally Posted by 3catcircus

Anyone know how Stage III ballistics are calculated? The math doesn't scare me, I just need to understand how the formulas vary in each range band.

As best I can reconstruct from poorly-annotated spreadsheets... use the following formula to generate the effective (for certain values of "effective") velocity for the range band:

(Vi + (2 x Vm) + Vo) / 4

Vi = velocity at inner boundary of range band
Vm = velocity at midpoint of range band
Vo = velocity at outer boundary of range band

Thus, for the Medium range band (100m to 200m), you'd use

(100m velocity + (2 x 150m velocity) + 200m velocity) / 4

- C.

[3catcircus]

Thanks! Seeing this - it makes sense that a weighting factor was applied towards the middle while averaging the velocity across the range band.

Originally Posted by Tegyrius

As best I can reconstruct from poorly-annotated spreadsheets... use the following formula to generate the effective (for certain values of "effective") velocity for the range band:

(Vi + (2 x Vm) + Vo) / 4

Vi = velocity at inner boundary of range band
Vm = velocity at midpoint of range band
Vo = velocity at outer boundary of range band

Thus, for the Medium range band (100m to 200m), you'd use

(100m velocity + (2 x 150m velocity) + 200m velocity) / 4

- C.