The Relative Burning Speed of Smokeless Powders By Roberto
Serino
Unfortunately, reloaders can't do without some mathematics, but
if you have understood the specific impulse concept, you are ready
to get more. The Du Pont Index is a very useful figure in order to
choose the right powder. It's seven times the Relative Quickness,
or seven hundred times the ratio between the Energy Potential
(ft.lbs/gr) and the Specific Impulse (lbs/sq.in*fps):DPI = 7*RQ =
700*EP/SIAs a matter of fact, it is not exclusively connected to Du
Pont powders, but it can apply to other powders, too. In this way
it can be even more useful, a true comparison being possible,
whereas manufacturers throughout the world still use their own
indexes.Concerning Du Pont powders, IMR 4350 is rated at 100; 3031
is rated at 135, while 4831 is rated at 95. 4198 is rated at 160;
between 3031 and 4350, you can find 4064 rated at 120, 4895 rated
at 115 and 4320 rated at 110. Finally, at the extremes, 4227 is
rated at 180 and is the fastest rifle powder and the slowest pistol
powder at the same time, while 7828, rated at 84, is the slowest
rifle powder.Now let's have a look at the IMR Handloader's Guide,
listing data about the .30-06 cartridge loading, with reference to
the standard 4350 powder, and to the heaviest bullet (220 gr.), so
that the propellant could be thought of spreading its full specific
impulse:30-06 Cartridge Load DataStarting Loads Maximum Loads
Bullet Weight (gr.)Bullet Dia.C.O.L.Powdergrs.Vel.
(fps)Pressuregrs.Vel. (fps)Pressure
220 GR. HDY JRN.308"3.230"IMR 782853.0228848,600
PSI58.0247659,700 PSI
220 GR. HDY JRN.308"3.230"IMR 483150.0227547,300
PSI54.0243857,200 PSI
220 GR. HDY JRN.308"3.230"IMR 435049.0228749,500
PSI52.5242557,300 PSI
220 GR. HDY JRN.308"3.230"IMR 406443.0224152,200
PSI45.5232556,700 PSI
First of all let's calculate the Sectional Density (lbs/sq.in)
of the bullet; in this case the Bullet Weight (gr.) will be divided
by the actual cross section area, not the simple squared Diameter
(in):-SD = (4/p)*(BW/7000)/D2 = 1.27*(220/7000)/(.308)2 = .422
lbs/sq.inThe specific impulse being unknown, it only can be
estimated by the maximum load velocity. In this case it can be
supposed the load has been completely burnt when the bullet leaves
the muzzle of a barrel 22" long.The burnt gases momentum and the
friction along the bore must be taken into account, too, because
burnt gases propel themselves as well as the bullet, and the
friction acts against the bullet motion.So let's multiply Sectional
Density by Muzzle Velocity developed at the maximum load
conditions; then increase the result with half the ratio between
Powder Weight and Bullet Weight, and with a Friction Coefficient
(which can be extimated about .2 for rifled bores, according to
Hatcher's Notebook):SI = (1 + FC + PW/BW/2)*SD*MV = (1 + .2 +
52.5/220/2)*.422*2425 = 1350 lbs/sq.in*fpsThe meaning is: if you
want to drive a bullet having a sectional density of .422
lbs/sq.in, at 2425 fps, in a barrel 22" long, the powder burning
behind it must provide a specific impulse of 1350 lbs/sq.in*fps at
least.Physically, IMR 4350 is a single-base, extruded, tubular
propellant (as are most Du Pont IMR powders). Its particles are
made of colloided nitrocellulose, coated with DNT to retard
burning, and heavily graphited to flow better through powder
measures.Its Energy Potential is about 1,246,000 ft*lbs/lb, or 178
ft*lbs/gr (according to the NRA Firearms Fact Book), so that
Relative Quickness and Du Pont Index can be calculated:RQ =
100*EP/SI = 100*178/1350 = 13.2 (extimated) DPI = 7*RQ = 92.3
(extimated)This result is less than 100, due to the fact that
probably the actual Specific Impulse is a bit smaller than 1350, or
the actual Energy Potential is a bit greater than 178.
Nevertheless, as you can see, it's very close to the IMR 4350
standard rating.All of the powders in IMR family have
approximatively the same Energy Potential, so their Specific
Impulse can be derived simply dividing this figure by their Du Pont
Index, and then multiplying the result by 700:SI =
700*EP/DPISubsequently the above formula yields to the following
estimated value for the specific impulse, from fastest to slowest
(SI estimated):Powder.....DPI....RQ....SI
(est.)IMR4227...180...25.7...692IMR 4198...160...22.9...779IMR
3031...135...19.3...923IMR 4064...120...17.1...1038IMR
4895...115...16.4...1083IMR 4320...110...15.7...1133IMR
4350...100...14.3...1246IMR 4831...95...13.6...1312IMR
7828...84...12.0...1483Alliant Powders (formerly Hercules Powders)
are double-base, extruded propellants, with nitroglycerin ranging
from 4% to 40%. Some of rifle powders are coated with DBP to retard
burning. Alliant own index is just Relative Quickness as defined
above, rating as follows from fastest to
slowest:Powder.......DPI...RQ2400............189...27.0Reloder
7....136...19.4Reldr 10X...118...16.9Reloder 15...96...13.7Reloder
19...79...11.3Reloder 22...78...11.1Reloder 25...74...10.5According
to the tables above, IMR and ALLIANT rifle powders can be located
as follows:Powder.......DPI...RQ2400............189...27.0IMR
4227....180...25.7IMR 4198....160...22.9Reloder 7....136...19.4IMR
3031....135...19.3IMR 4064....120...17.1Reldr 10X...118...16.9IMR
4895....115...16.4IMR 4320....110...15.7IMR
4350....100...14.3Reloder 15...96...13.7IMR 4831.....95...13.6IMR
7828.....84...12.0Reloder 19...79...11.3Reloder
22...78...11.1Reloder 25...74...10.5The matter seems quite simple,
but unfortunately DPI and RQ are not the only tips for choosing the
right powder, because bulk density, specific gravity and case
capacity play a role as well as bullet sectional density and
expected muzzle velocity (in a word, the specific impulse).So
various Burn Rate Charts display powders in a somewhat different
order, taking into account the bulk density of powders, their
specific gravity and the average case capacity of peculiar
cartridge loading.This happens especially to powders showing very
close value of RQ, like IMR 4064, 4895 and 4320, or Reloder 15 and
IMR 4831, or IMR 7828, Reloder 19, Reloder 22 and Reloder 25, or
Unique, Herco, PB and SR 7625.Just for comparison, find here DPI
and RQ of pistol and shotgun powders by IMR and
Alliant:Powder.......DPI...RQBullseye.......700...100Red
Dot.......659...94.1700X............635...90.7Green
Dot....545...77.9Unique.........431...61.6Herco...........393...56.1PB................390...55.7SR7625.......340...48.6SR4756.......305...43.6Blue
Dot......265...37.8SR4759.......210...30.0WARNING! NEVER determine
a load from this, or any burn rate chart. Employ only the newest
published data for the specific powder you intend to use.
