How you guys understand Ballistic Eficency?

Greetings

Peter

Let me count the ways. I'm not sure that there is a singular significant meaning to balistics efficiency. roger

I've a simple definition of "ballistic efficiency"...if the bullet goes where I want it to go and does what I want it to do, then it is "efficient"...

PeterPan,

I beleive that the most common way to look at ballistic effecincy is to calculate
fps/grain of powder.

This may have significance to a military cartridge, but for a hunter results at the game are the most important. IMO

BigBullet

I got over "ballistic efficiency". It happened when I used to have the best chuck rig in the group which was a .222. Then we got more money and cars and started to drive to NY to shoot chucks at longer ranges.

So there we were and it was a long shot. My buddy said "here, use my 22-250". That was the end of "ballistic efficency" for me!

To argue the other side the Late Charles Landis the author of many fine books on varmint rifles mentioned cartridges of low report over and over. It must still matter now.

I get around that by carrying reduced loads and getting closer.

The general notion of efficiency is output/input, i.e., how much useful output you get for a given amount of input. The efficiency of an electrical transformer is power out/power in.

I'm not sure that in shooting that convention is followed. If it is, then it would be (kinetic energy of the bullet)/(chemical energy stored in the powder).

A completely separate issue is effectiveness. A 22-250 is not very efficient, but is wickedly effective.

What about this:

Ballistic Efficiency Ability of a projectile to overcome the resistance of the air. Ballistic efficiency depends chiefly on the weight, diameter and shape of the projectile.

Peter, what you're talking about is usually termed "ballistic coefficient."
And for that, you have a pretty good definition.
Strictly speaking, ballistics is the study of flight characteristics, and so ballistic efficiency would be a measure of how easily a projectile could be made to exhibit some characteristic. A long bullet would maybe be more efficient at delivering X ft lb of energy to a target than a short one, perhaps, so your definition could certainly be applied.

Some fellow in some magazine I browsed at Barnes and Noble came up with an interesting concept of efficiency:

Ft/lbs on target per grain of powder at xxx yards.

This concept balances the decrease in internal ballistic efficiency of the smaller bores with their increased external ballistic efficiency.

For hunting at least, the ability to do work at the target is really what counts.

It helps that he found that the 35's did best in this regard! Gotta love those 35's. FWIW, Dutch.

.

For EXTERNAL ballistics, efficiency is measured by the ballistic coefficient, as noted by others.

For INTERNAL ballistics, efficiency CAN be measured by charge weight vs velocity or energy, but this is an absolute and not a relative measure. Hence the fact that a 222 is far more efficient than a 22-250. At a constant pressure limit, a bigger case will ALWAYS be "less efficient" than a larger one for a given bullet diameter in an absolute sense.

However, RELATIVE efficiency is determined by comparing how much kenetic energy SHOULD be delivered by a given case capacity and bullet diameter compared to that which is delivered by a particular load using the same capacity and diameter, holding pressure constant.

For any bullet/bore diameter, energy potential is a function of the case capacity raised to the 0.6 power. Hence, if you double case capacity you get 2^0.6 times more energy, or 1.52 times as many foot pounds at the muzzle.

For example, a .308 win with a 50 grain capacity taken to 52,000 CUP will get you 2850 FPE. If you expand the case to a 100 gr capacity, you SHOULD get 1.52X2850 or 4332 FPE. If you get more energy than that, you are more efficient than the 308 on a relative basis (assuming that you use enough bbl length for each test).

You can do the same thing when holding case capacity constant and changing bullet/bore diameter, but the exponent is twice that used for capacity since bore area is a function of the square of bore diameter. Since X^2 raised to the 0.6 power is X^1.2, the case capicity exponent is 1.2.

For example, if we squeeze the 308 win down to .243 we get (.243/.308)^1.2 or 0.75 time the energy of the 308. 0.75 times 2850 is 2137 FPE, which is what we SHOULD get from the 243 win.

Now, the key to all of this is to use the powder that gives the max FPE in each case. The 308 might use R15 while the 243 might use R22. It is a simple matter to use these same factors to estimate the best burn rate and bbl length for any case capacity/bore combination and viola! you have your own internal ballicitcs calculator.

To the extent that you fall below or above the estimated energy level, you are either less or more efficient on a RELATIVE basis.

For a relative measure

What about this way at looking at efficiency?

A given gun/cartridge combination is constrained to a maximum working Pressure. For a given max Pressure, a max Force can be applied to the bullet: Force = Pressure * area of the bullet base.

From this max Force, a max Acceleration can be calculated (Accel = Force / mass).

Assuming this max Force is applied down the entire barrel length, and neglecting friction, the max velocity of the bullet can be calculated: max Velocity = Sqrt (2*max Accel*barrel length).

The efficiency would then be the (actual velocity/ max theoretical velocity). Which cartridges are most efficient using this definition?

Washougal Chris