Back in the old days, there was original gunpowder – black powder. The combination of sulfur, charcoal and saltpeter (potassium nitrate) is quite old and fairly simple – although dangerous at times – to use. Black powder is termed a ‘explosive’. That is, it always burns at the same rate, whether confined or open to the air. It is a ‘low’ explosive in that the burning rate is subsonic and it is not as brusque, so to speak, as ‘high’ explosives.
But black powder was used in all sorts of guns. Everything from rather small caliber handguns to cannons. Due to the constant burning rate, it cannot be over-loaded. When it goes off, it simply burns and the excess is expelled from the gun. The exception being if the gun will not hold the basic pressure of the initial burn. But then, even a minor load of black powder will damage the gun.
By the 1800s, gunmakers and shooters found by sizing the kernels or granules of the powder, the burning rate is somewhat effected. Essentially, the smaller bits tend to burn ‘faster’. This is still limited to the overall limitations of the chemical compound, but is somewhat useful for different applications. This is recognized by the designations of “Fg” which means ‘fine granule’ and “FFg” (fine, fine granules) and so on. One uses the smaller sizes in shotguns and handguns.
Then ‘smokeless’ powder happened. Actually, modern smokeless powder was developed in stages. There are a couple of places on line which explain the development of smokeless powder and I’m not going to copy it here.
The benefit – and complication – of smokeless powder is smokeless can be better suited by design for certain firearms. The benefit of course is that a specific application makes the application more effective. The complication is selecting which specific powder for a specific application.
Just for the record, smokeless powder is a propellent, NOT an explosive. Properly used, smokeless powder of any ilk does not explode. It burns very quickly when confined, but this is NOT an explosion. The term ‘explosion’ gets used often by the press (in ignorance), fiction writers (also in ignorance), and various other official people of one level or other out of simple neglect (they ought to know better and don’t bother.)
When smokeless gunpowder ‘explodes’ the burning rate is much higher than the formulation is designed to function. This normally is destructive to some level to the firearm or cartridge and is rather dangerous. It is equivalent to what used to be referred to as ‘knock’ in a gasoline engine.
In general, the ‘specificity’ of any smokeless powder (‘any’ referring to the common use name of the powder) is the burning rate. Burning rate of smokeless powder is roughly similar to the ‘octane rating’ of gasoline. Just as higher octane gasoline burns slower and is more suited to high compression engines, ‘slower’ burning rate powders are more suited to higher pressure and velocity arms. Again, similar to gasoline, although the burn rate varies with specific formulations, all gasoline when confined burns far too quickly to differentiate with the naked eye; the same with smokeless powder. Virtually all the powder in a cartridge burns prior to the projectile leaving the barrel; mostly within the first few inches of the chamber and barrel. “Virtually” is used here as some small percentage of powder never burns. This is true of small handguns and large cannons. (It’s easier to see the unburnt kernels from cannon.)
Just for the record, the ‘muzzle flash’ of most arms is NOT the final burn of gunpowder not consumed in the barrel. See http://www.navweaps.com/index_tech/tech-090.htm for a full explanation.
Smokeless powder burns best when pressures developed are within certain (powder specific) ranges. By ‘best’ one means more uniformly. Generally, ‘faster’ powders burn at a lower pressure level than ‘slower’ powders. Therefore, ‘faster’ powders are typically used in lower pressure arms. However, this can be over done; I’ve found some cartridges and applications stretch the meaning of ‘slow’ and ‘fast’ at times.
There are several conditions which determine the proper burning rate for a specific application. No single condition is the sole controlling factor, it is a combination of all factors.
1. Ratio of bullet weight to powder charge. In short, the larger the powder charge relative to the projectile, the slower the burn rate of the powder. Consider the amount of Bullseye powder safely used in a .44 Magnum revolver compared to the amount of 2400 powder in the same revolver using the same bullet. Also consider the amount of any sort of powder used in a .357 Magnum to propel a 150 grain bullet versus the amount of any powder used to propel a 150 grain bullet from a .30-06 rifle.
2. Resistance to movement of the projectile. The more pressure needed to move the bullet requires a slower burning powder. Consider the demands of shooting a 200 grain .38 Special bullet against the demands of shooting a 200 grain .45 ACP bullet. The 200 grain bullet in .357 caliber is ‘heavy’, yet a 200 grain .452 caliber bullet is ‘moderate’.
3. Expansion ratio of the arm. Expansion ratio is the ratio of the initial burning chamber volume (cartridge in chamber with bullet in place) to the total volume of the chamber and barrel out to the muzzle (where the bullet no longer confines the expanding gasses. The larger the expansion ratio, the faster the powder need be. Check any loading manual. A rifle with a bottle neck cartridge (.30-30, .22 Hornet, .308 Winchester, .30-06 Springfield, 7mm Remington Magnum) requires a slower burning powder than a .458 Winchester, .45-70 Government [strong action] or .450 Marlin).
4. Pressure limitation of the firearm. Combined with the above relationships, some firearms are limited more than others in absolute pressure levels. For instance, a .380 ACP pistol operates at 21,500 psi while a .30-06 Springfield operates at 50,000 (CUP). Obviously, one must load different pressures in each.
This explains why so many ‘handgun’, ‘light rifle’ and ‘shotgun’ powders overlap. Both shotguns and handguns have large expansion ratios. Both shotguns and handguns have relatively smaller powder capacities. (Visualize a bullet or shot load moving down a barrel from chamber to muzzle. With each inch of travel of the projectile, a relative large bore will produce more volume than a relatively smaller bore. If the projectile outpaces the burn rate of the powder generating pressure, the pressure level of the firearm drops and velocity is limited.)
Unique is widely used as both a shotgun and a near universal handgun powder. Additionally, Unique may be used in many reduced (low) velocity (not to be confused with low pressure) loads in centerfire rifles. H-110 can be used in the M1 carbine and also for heavy loads in the larger Magnum revolver rounds.
Please note: Barrel length has nothing to do (directly at least) with prospective choice of powder burning rate. I keep seeing people on internet forums going on about how ‘short barrels’ waste powder as the powder doesn’t burn. Since the shorter barrel tends to lose velocity compared to a longer barrel, some feel a faster powder will either counter act the loss in velocity or at least substitute in a shorter barrel. Try it if you must, and chronograph the results.
Lacking a chronograph, study the findings in a loading manual. Note the absence of any comments regarding substituting a faster powder in any load due to a shorter barrel.
The fastest load in a given length barrel will be the fastest in any other length barrel. This ‘short barreled’ rifle may in some circumstances may shoot faster than a different ‘long barreled’ rifle, but the fastest load combination in either of the rifles will be the fastest in the other as well. This ‘short’ versus ‘long’ effect may be more noted in handguns; but the relative velocities remain intact. (One rifle may prefer a different load for accuracy, but rifles are like that. Accuracy and velocity have NO absolute correspondence. In my experience, I’ve had very accurate ‘fast’ loads and very accurate ‘slow’ loads. There is no cosmic rule linking velocity to accuracy. That’s been discovered and announced yet, anyway.)
Good news. The typical reloader does not have to figure this all out by one’s self. The loading manuals have done all this in their research, even if they don’t specifically mention it.
When one buys a loading manual, read all the expository and explanative parts BEFORE jumping into the loading data.