The Relative Burning Rate of Powders
Most reloaders wonder whether burn rate charts are really meaningful, or meaningless. As a matter of fact, burn rate charts are commonly compiled by propellant manufacturers, in order to give some directions for use to the guys who like to reload their own ammunition. Do you think that manufacturers are all wrong?
I don't say they are always all right, but certainly they do know what they are doing. Randy Wakemann is correct when he says professional ballisticians don't use relative quickness for load development; but that is because figures derived from closed bomb tests aren't the same thing as the relative burn rate we are discussing.
Obviously you have to take into account primer, case capacity, loading density, bulk density, sectional density, and so on, in order to develop the right load, and then you must test it in a pressure gun.
On the other side, the relative burn rate as presented by powder manufacturers is just a rank number, and the informations it provides is useful to make the right choice among their powders. Some troubles occur when different brands of powders come in comparison. Each manufacturer does use his own index, and Winchester ones are not the same of IMR or Alliant or Accurate.
Many students and shooters have tried to put current powders in the right order, starting from the highest burning rate and ending with the slowest powder. The task is not easy, because different brands of powders show different ignition characteristics, energy content and composition, so many powders can be located only approximately. Moreover, common canister powders show some differences from lot to lot, and this adds more confusion to the matter.
A good solution can be found to the question by carefully reading the shooting data from reloading tables, which are compiled on the basis of reliable pressure gun tests. Such data is given in the same units, that is to say bullet diameter in inches, bullet and powder weight in grains, and muzzle velocity in feet per second.
Does this data have anything to do with the relative burning rate of powders? Yes, it does, as you will soon see.
You know that current barrel lenghts rate 4 to 6 inches for handguns, 20 to 24 inches for rifles and 26 to 32 inches for shotguns. You know, too, that the slower the powder, the higher the load, for a given muzzle velocity in a given barrel length of a given bore diameter. Moreover you know that the quicker the powder, the higher the velocity, for a given powder load and a given bore diameter. Finally, you know that the larger the bore, the quicker the powder for a given muzzle velocity and a given powder weight.
So it seems meaningful to combine reloading data, as they are widely known, for finding a working figure about the Relative Burning Rate. How can we combine such data?
Every shooter should know about sectional density, as referred to bullet weight and squared diameter; this figure can be referred to powder weight, too, giving the powder sectional density in grains per square inch.
Well, it can be proved that the Relative Burning Rate (we talked about it in the past) is roughly proportional to the ratio of Muzzle Velocity (fps) to Powder Sectional Density (grs/sq.in), as follows:
RBR = MV*D^2/PW
Now let's try to calculate RBR from IMR 30-06 cartridge reloading data. We know that a 52.5 grs load of 4350 can propel a 220 grain jacketed bullet at 2425 fps; first let's find the Powder Sectional Density:
PSD = PW/D^2 = 52.5/.308^2 = 553.4 gr./sq.in
Then let's calculate the RBR:-
RBR = MV*D^2/PW = 2425*.308^2/52.5 = 4.4
For comparison, let's do the same for a 58.0 grain load of 7828 providing 2476 fps:
RBR = 2476*.308^2/58.0 = 4.0
Again, the same for a 54 grain load of 4831 giving 2438 fps:-
RBR = 2438*.308^2/54.0 = 4.3
Finally, the same for a 45.5 grain load of 4064 achieving 2325 fps:-
RBR = 2325*.308^2/45.5 = 4.8
These figures look somewhat different from those we have seen the last time, speaking about the Relative Quickness; but if you multiply by 100 and divide by 32.175 (i.e. the gravity), you will soon realize that it's quite the same thing we have discussed before:
powder RBR RQ
4064 = 15.1 - 17.1
4350 = 13.6 - 14.3
4831 = 13.3 - 13.6
7828 = 12.6 - 12.0
Finally, if you multiply by 7 (as one pound contains 7000 grains), you will find a figure that is very similar to the Du Pont Index:-
powder RBR DPI
4064 = 106 - 120
4350 = 95 - 100
4831 = 93 - 95
7828 = 88 - 84
The differences are negligible, due to the different method of calculation, and now we can say that RBR, RQ and DPI are almost the same thing. If you repeat the task for the same powder in different loads, you will find slightly different results, that anyway group around a mean value; this mean value can be assumed as the powder rank number.
All of IMR rifle powders are extruded single-base. Let's repeat the task for Winchester rifle powders, that are double-base spherical ones, in the same 30-06 cartridge.
We know that 49.0 grs of W760 accelerate a 220 grain jacketed bullet up to 2370 fps:
RBR = (2370*.308^2/49.0)*(100/32.175) = 14.3
Again, 45.0 grains of W748 propel a 180 grain bullet up to 2540 fps:
RBR = (2540*.308^2/45.0)*(100/32.175) = 16.6
Now we can say that W760 is a bit faster than IMR4350; similarly W748 is a bit faster than IMR4064; moreover, W748 isn't very suitable for 220 grain bullets.
What do you think about shotgun and handgun powders? Well, 17.5 grains of IMR 700-X give 1200 fps in a 1-1/8 oz. 12 ga. load:
RBR = (1200*.729^2/17.5)*(100/32.175) = 113
With the same components (primer, shell, wad and lead shot), 20.0 grs of WST also give 1200 fps:
RBR = (1200*.729^2/20.0)*(100/32.175) = 99.1
This means that WST is slower than IMR 700-X.
Finally, 4.6 grs of IMR PB behind a 170 grs jacketed bullet give 920 fps in a .357 Magnum load:
RBR = (920*.357^2/170)*(100/32.175) = 79.2
In the same cartridge 7.0 grs of W231 behind a 150 grs jacketed bullet give 1269 fps:
RBR = (1269*.357^2/150)*(100/32.175) = 71.8
This means that IMR PB is faster than W231.
So, as you can see, the numbers given by manufacturers and ballisticians aren't meaningless; they mean just what they are figured from. That is to say that if you want to propel a bullet of a given diameter at a given muzzle velocity and with a given powder weight, you have to choose a powder that display a RBR not greater than the ratio of muzzle velocity to powder sectional density.
The rank number is meaningful mainly with regard to single loading categories (i.e. shotgun, handgun or rifle), because of different thermal efficiency of the loads. The same powder, loaded both in shotgun and in handgun cartridges, may show different RBR values.
Unfortunately these figures don't say anything about the breech pressure. They are only useful to locate different powders on the basis of loading performance.
Copyright 2007 by Roberto Serino. All rights reserved.