Making Magnums move!
The significance of IMR 4350's debut was that it was the first genuinely slow-burning progressive propellant canistered for handloaders, and it made the postwar avalanche of new magnum cartridges feasible. In addition, it gave new life to many older cartridges, rounds such as the 7X57mm Mauser, .257 Remington Roberts, the wildcat .25-06, .300 H&H Magnum, and even the .30-06 with heavy hunting bullets.
The significance of H4831's appearance was that it could be bought in bulk, in those early days, for as little as 50 cents a pound. Even though a penny was a penny in 1946, that was dirt-cheap, and opened metallic handloading up to the multitudes. After all, a pound of IMR 4350 nicked the budget for all of $3.95 in that dear, dead day. With H4831, the powder in a typical handload cost less than a half-cent, which, with the availability of military brass and good jacketed bullets at about four cents each, made it possible to shoot an 'ought-six for maybe a nickel a roar. A fellow could pay for a press and a set of dies pretty quickly out of his shooting savings at that rate. After the war years, the reloading hobby was ready to take off anyway, but 4831 unquestionably helped fuel the liftoff.
If slow-burning propellants almost literally launched modern metallic-cartridge handloading, then it may be worthwhile to take a hard look at them, as a group and individually.
First, the term itself. "Slow-burning" is actually a bit of handloading lingo, having little precise meaning. In using such jargon, "-burning" is said to refer to the Relative Quickness (RQ) of the powder being discussed, and RQ is a scientific concept describing the rate at which a burning gun-powder yields its latent energy to some other powder being used as a reference standard. For example, on Du Pont's RQ scale (which happens to use IMR 4350 as the reference, at an arbitrary rating of 100) IMR 4064 rates at 120, 3031 at 135, 4198 at 160, and 4227 at 180. On the same scale, some Du Pont shotgun powders may rate as high as 675. What it means is simply that IMR 4227 releases energy at a rate which is 80 percent faster than that of 4350, as determined experimentally in laboratory closed-bomb tests.
Unfortunately, there is no universal relative quickness scale on which all powders of all manufacturers can be accurately related to each other in this manner. Such amalgamations have been attempted and are published from time to time (currently on page 648 of the Hornady Handbook of Cartridge Reloading, 3rd Edition), but such a listing must be of largely academic interest. It states that Winchester-Western 680 is fastern-burning than Norma 200, which is true enough, but the comparison is not quantitative; it cannot tell you how much faster, and any effort to interpolate actual handload charge weights from such a table is foolish.
A couple of other problems exist in describing a "slow-burning" powder. One is that there is no universally-accepted definition of that term, and the other is that, in ballistical matters as in most other things, everything is relative. A powder which is "slow" in the .223 Remington case will obviously be very fast in a 7mm Remington Magnum. Straight-sided cases such as the .45-70 make powders behave very differently from bottlenecked brass, and, even in the same case, different bullet weights can made a difference. In other words, for the purposes of this article, the slow-burners are the ones I say belong to that group.
And I say that the slow-burning powders are Du Pont IMR 4350 and IMR 4831, Hodgdon's H4350, Spherical H450, H4831, and Spherical H870, Norma 204 and MRP, Winchester-Western 785 Ball, and Accurate Arms' MR 3100 and MR 8700, both ball type. Discontinued propellants in the same class of which small quantities may still be found are Hodgdon's H205, Norma's 205, and possibly Hodgdon H570, Olin 780 BR, or even the original, war surplus lot of H4831. Some readers may wish to dispute my arbitrary drawing of the line, claiming that H414, W-W 760, or even IMR 4320 belong with the slow-burners. If so, I'll certainly not argue the point, but my remarks about slow-burning propellants are intended to apply to those listed above, and to no others.
One reason for my selections is that the rifle powders listed are the ones for which the so-called "magnum" primers are most often recommended, and another is that these are the only propellants which ever seem to have been associated with the mysterious paradox of destructively high pressures resulting from reduced loads in certain cartridges.
This is sometimes called a "secondary explosion effect" (SEE), a pressure excursion, or simply "detonation", and although its very existence was the subject of a raging debate in handloading literature of a couple decades, it is both real and very dangerous. Some learned papers have been written to explain the physics of a pressure excursion, and a number of theories have been advanced. Not being a physicist, I'm not certain that I understand them well enough to discuss them, or to judge which one--if any--is the true explanation, but I do know that the phenomenon does occur and I'm informed that it has even been experimentally reproduced, more or less at will, in a major ballistics laboratory.
All the handloader needs to know about such pressure excursions, however, is that they may occur when three factors are all present in a reloaded cartridge. Thos factors are: (1) an "overbore" case, which is one with a large powder capacity relative to bore diameter (see below); (2) a slow-burning propellant; and (3) a charge weight reduced by more than about 10 percent from maximum. All three factors apparently must be present, and, even then, a pressure excursion is quite a rate phenomenon, fortunately. The reloader can positively protect himself from it, according to the experts, by making certain that loads of slow-burning propellants are not reduced by more than 10 percent from handbook maximums. If a reduced loading is wanted, simply switch to one of the medium-burning powders (say, IMR 4320 or quicker) offering the desired ballistics.
The cartridges which appear to be most susceptible to pressure excursions when all the other factors are present (and only then) are the belted smallbore magnums (6mm, .25, .270, 7mm, and .30) and such as the .25-06, .270 WCF and various wildcats in the .22 to .25 caliber range on the .30-06, .284 WCF, and similar cases. I have heard of a few suspected pressure excursions in the non-belted 6mm cartridges, too. This is not to say that there is anything whatsoever wrong with the use of full charges of slow-burning powders in any of these cases; on the contrary, with many bullet weights the slow-burners are the ideal powders in such cartridges. It is merely that greatly reduced loadings of such propellants should be avoided in them.
With that out of the way, we can move on to other characteristics of this group. It will be noticed that they are of two types, extruded (or "stick" powders) and "Ball" (a Wincester-Western copyrighted designation) or "Spherical" (a Hodgdon trademark). Among the extruded numbers, only the Norma powders are double-based, meaning that they have a small percentage of nitroglycerine added to the basic nitrocellulose to increase latent energy. All ball-type powders are double-based.
Burning rate can be controlled in many ways, by chemical content, granule size and shape, perforation of extruded powder grains, and by coatings which deter combustion somewhat. All are used with the existing extruded powders, while in the ball types burning speed is controlled by granule size and coatings, especially the latter. The granules of slow powders tend to be much larger in both types than in faster-burning propellants, which, together with the heavy deterrent coatings, tend to make these granules more difficult to ignite; hence, the recommendation of high-brisance, or "magnum" primers which produce more flame volume of longer duration for positive ignition, especially with the relatively large powder charges common on magnum cartridges.
The larger grains of the extruded types (often called "log" powders), can create headaches in powder measures, due to their tendency to bridge and hang up a portion of the charge in the drop tube. Measured charges with them usually reveal a somewhat larger variation in weight than small-granuled numbers. On the other hand, the slow-burning ball types flow through a measure almost like water, producing excellent uniformity. The density of ball-type powder charges in often useful, too, in allowing the handloader safely to pack more energy into the limited space available in some case designs than is possible with most extruded types. The short-grained, dense, double-based Norma propellants are an exception among the extruded ones, however.
The major virtue of all the slow-burning powders, and the reason for their existence, is simply that they evolve their gases for propulsion of the bullet in a more controlled, progressive manner, continuing to burn longer as the bullet moves along the bore. This characteristic has several advantages. First, because pressures build more gradually, the projectile is started less violently, with a better chance of engaging the rifling in good alignment. This can only improve accuracy potential, especially with the heavy bullets with which the slow-burners are most often used. Second, acceleration continues through bullet travel within the bore (or nearly so), so higher velocities are attained without unacceptably high breech pressures. Obviously, longer barrels can take greater advantage of this more prolonged push than short ones, and, in fact, very short barrels may partially negate the potential of slow-burning powders, burning some portion of the charge in the atmosphere after the bullet has departed.
With this brief description, we can begin to assemble a picture of the typical application for slow-burning powders. It will involve large-capacity cartridge cases-large, that is, in proportion to bore diameter, wherein faster-burning powders would create dangerous pressures in high-density loadings, or leave too much empty space with charges producing normal maximum pressures. The slow-RQ numbers are particularly valuable with long, heavy bullets in such cartridges, bullets which offer high inertial resistance to movement and which accelerate more slowly than lighter slugs.
By way of illustration, the two mightiest magnums in common use today are the .375 Holland & Holland and the .458 Winchester, but neither of these is at its best with the slow-burning powders, despite their heavy slugs. The reason is that their calibers are quite large in relation to case capacity. The magnums which make the most of the slow-burners-which, in fact, couldn't exist in their popular forms without them--are the smallbore magnums, typified by such as the .300 Winchester or Weatherby and the .264 and 7mm Magnums. And it is with their heavier bullet weights that even these come into their own with the slow propellants.
Many non-belted cases also benefit from these powders, however. For example, the .25-06 (developed in wildcat form in the mid-1920s) really did not have any suitable propellant powder until IMR 4350 became available in 1940, after which it began to sing! The 6mm twins--.243 WCF and .244/6mm Rem.--might never have existed in the absence of the slow-burners, and much of the original controversy over the merits of the .270 WCF would have been moot if they have been available when it was introduced in 1925 (they added at least 150 fps [feet per second] to the velocity of the 130-grain bullet, at lower pressures, and did even more for its heavier slugs). On the other hand, if slow powders had been available when the .257 Roberts was originally wildcatted, there may not have been a niche for the 6mms to fill.
The bulkiness of the slow-burners also represents a safety factor of sorts, making accidental double charges unlikely or impossible in even the largest cases, and in many of the smaller-capacity cases even preventing a serious overcharge. A reloading buddy of mine, back when H4831 first appeared, was wont simply to dip 8mm Mauser cases full and seat a bullet! These loads produced remarkably uniform ballistics and excellent accuracy and killing power. I must emphasize that this story is not to be taken as a recommendation for a handloading shortcut, and such a practice, with a different case or powder, could have disastrous results ... but it does illustrate the point.
A dangerous confusion sometimes arises from the similarity of trade names carried by different powders, and it's especially prevalent among the slow-burners. We have IMR 4831 and H4831, IMR 4350 and H4350, Norma 205 and H205, and none of these pairs have exactly the same relative quickness, meaning that loading data are not interchangeable. IMR 4831 is distinctly faster-burning than H4831, and substituting the former for the latter in a maximum load will inevitably produce an overload.
To make matters worse, neither of the current, newly-manufactured 4831s is quite as slow as the old, original, surplus lot of H4831, for which so many loading formulas have been published over the years. The point is that the reloader must read published loading data very, very carefully, to make certain that he understands exactly which propellant is being recommended; those "IMR" or "H" prefixes make a lot of difference. I believe it would have been a service to the handloader (and will be, when similar decisions are made in the future) if the manufacturers had chosen unmistakably different numerical designations to avoid confusion.
In general, the world of slow-burning powders is the world of high velocity. Almost without exception, the highest-velocity big-game cartridges in the world are at their best when fueled with the slow-burners, and, almost without exception, these propellants are behind the fastest bullets from either handloads or commercial ammo-although powders used in factory ammo are not exactly the same as the canistered powders sold for the same purposes. It can be said that the history of progress in powders from black powder days has been a story of development of slower and slower gunpowders, producing higher and higher velocities within the standard limitations of case capacity, rifle strength, breech pressure, and recoil. Powders have certainly been improved in many other ways, too, but the most striking advance to data has been in slower burning rates.
For a given cartridge case and bullet weight, a propellant powder really cannot be too slow ... until that point is reached where the handloader cannot stuff enough of it into that case to develop maximum safe working pressures. In other words, whatever the rifle cartridge, the slowest suitable powder can be expected to produce the highest muzzle velocity, and that's what most handloaders are seeking, most of the time, in most of their guns.
High velocity, of course, means flatter trajectories, less wind drift (normally), and more retained striking energy downrange. Those are the benefits of slow-burning powders. There are drawbacks, as well. They include higher powder costs, more trouble in measuring, fewer loads per pound, more recoil, and, commonly, more rapid bore erosion. In reloading as in most other activities, you don't get something for nothing, but, on balance, the slow-burning powders have perhaps been the greatest boon to riflemen since the invention of the breechloader.
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|Title Annotation:||evaluation; IMR 4350 gun power introduced in 1940 by Du Pont, a first in slow-burning canistered powder for handloaders|
|Publication:||Guns & Ammo|
|Date:||Feb 1, 1985|
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