I call it "Efficiency Ratio"

[Rambozo]

I don't know if this is the reinvention of the wheel or not, though I imagine that it might be useful in either case.

The idea here is to use one standard of measurement for projectile comparison regardless of weight, profile, or caliber. For that, I've come up with what I call the "Efficiency Ratio", which becomes the lowest common denominator across projectiles.

It works like this.
ER(Efficiency Ratio) = ((Ballistic Coefficient*1000)/Weight)*Diameter

Ok, so I multiply the BC by 1000 only to produce a manageable number, then divide that by the bullet weight in order to determine the profile efficiency (Higher BC to weight ratio = more efficient profile). The need to multiply the diameter only acts as an attempt to take into account the chamber pressure necessary for velocity, so that relates to use of case capacity (projectile of larger diameter to weight ratio would reach velocity with lower pressure).

So by converting this to a single number, bullets could then be compared simply by this Efficiency measurement, and that might help in terms of bullet/cartridge selection.

So some examples.

The 123gr .264 Hornady Amax with a BC of .510 would have an ER of 1.09

The 155gr .308 Sierra Palma Matchking (2156) with a BC of .473 would have an ER of .9399

So that means then that the 123gr Amax should then perform better than the 155 match king in terms of trajectory from a similarly classed case (.260 Rem).

It is important to note however, that this is only a measurement of efficiency, and so doesn't necessarily mean much in terms of wind drift or actual practical use of the projectile/cartridge. A .308 cannot practically shoot as far as a .50bmg, and a higher ER won't do much to change that. But it might make selection of components easier across similarly classed cartridges.

It's also important to note that it really doesn't matter what BC profile (G1, G7, etc.) you use, provided that the comparison doesn't go across the profiles of course. It would be best then to specify the profile used.

What say you? good idea?

[Lead Chucker]

I like it! Makes since.

[rcvisket]

I like where you are going with this but the inertia of the projectile also determines stability. that is why SMK have variable BC rather than just one. AMAX give only one BC but generally the faster the bullet is traveling the more stable it is until it hit the trans-sonic region of it's flight...so i have gathered. but this would be an interesting formula to start working on. of course correct me i i am wrong. I actually noticed something like this earlier today going over load data. im switching back to 123 SMK instead of 130 berger VLDs because for my atmospherics it is 12.2 mils for 1000 meters where as the 130s were at 14.7mils.

[Rambozo]

...

yeah, I understand that bc isn't a constant, and that there is deviation, hence the multiple BCs you will often find. You will notice though, that the BC I'm using here, isn't that of what Sierra advertises. This BC is derived by taking an average from the practical velocity range (realistic muzzle velocity to soundbarrier) output of the JBM Drag and Twist Calculator. This BC has proven to work surprisingly well for me out to 1000.

The Hornady Published BC for the 123 has proven to be pretty accurate in my experience, so I just stuck with it.

Like anything though, I imagine that this calculation is only as good as the information you give it.

[rasp65]

Rambo It is interesting that a bullet like the 750 gr A-Max with a BC of 1.05 has a lower efficiency ratio using your formula. I know there are some new heavy .338 bullets that have BC over 1 that would also be lower using your formula. So I guess that I am not sure what the efficiency pertains to exactly.

Visket The reason the SMK has 3 BC's is because Sierra unlike most other bullet manufacturers spent the time and money to calculate the changing BC of bullets. So droptables can be calculated more precisely. On the other hand Berger uses the G7 profile and although lower than G1 numerically they produce more accurate tables for long range shooters.

[rcvisket]

So how does one calculate the variable G7s for an SMK?

[Rambozo]

Rambo It is interesting that a bullet like the 750 gr A-Max with a BC of 1.05 has a lower efficiency ratio using your formula. I know there are some new heavy .338 bullets that have BC over 1 that would also be lower using your formula. So I guess that I am not sure what the efficiency pertains to exactly.

Visket The reason the SMK has 3 BC's is because Sierra unlike most other bullet manufacturers spent the time and money to calculate the changing BC of bullets. So droptables can be calculated more precisely. On the other hand Berger uses the G7 profile and although lower than G1 numerically they produce more accurate tables for long range shooters.

That's because I am defining efficiency by weight and diameter relative to BC. So yeah, the 750gr amax has a really high BC, but it's also 750 grains. If it was lighter, while maintaining that BC, then it would be a more efficient profile.

The purpose here, is to find the projectile with the highest BC by weight, and caliber. So the output isn't to suggest that a more efficient bullet is necessarily any better than a less efficient one for a specific purpose, but to help find a better bullet when differences are more subtle. It's usefulness, I think, is limited to like cartridges and bullets. It's more aimed to finding the bullet with the best balance of BC, Weight, and Velocity potential.

A bicycle is efficient, but not very practical for the Dakar rally. The same logic holds true here. I'm not suggesting that efficiency is necessarily best. There will just be those scenarios where a cartridge just can't do some things, despite how efficient it is.

[Rambozo]

use an average. Any single BC you find published for any other bullet is just an average anyway. That JBM website is an awesome resource for finding BCs. You can also convert across the different profiles to find the one that gives the lowest standard deviation across the velocity range. That calculator is the Drag Function Array Converter