This essay is a basic primer on firearms propellants or gunpowder. One does not have to decide which powder to use from scratch; the loading manuals have worked out much of that. However, this goes into some detail about how such matters work and why ‘this’ powder is good for ‘this’ cartridge and ‘that’ powder works best in something else smokeless.
As long as the reloader scrupulously follows loading manuals, this essay may be ignored.
Ballistic Barrel Length: In any arm, this is the distance the bullet is ‘pushed’ by the expanding gases. It is measured from the base of the loaded and unfired bullet - not cartridge - to the muzzle. One ignores the length of cartridge and gap in a revolver. In handguns, the ballistic barrel length is slightly shorter for a semi-automatic or single shot pistol, and somewhat longer for a revolver.
Burning Rate: The ‘speed’ a particular type of gunpowder burns a certain weight of powder. This is tested in ‘closed bomb’ tests not practical for the individual. Burning rate can be changed crudely by the overall size of the kernel (not to be confused with ‘grain’, a unit of weight), perforations in the individual kernels (commonly in extruded types) and external coatings, which are chemical and designed to slow down the initial burning rate. (Black powder is somewhat modified by the size of individual kernel only; Fg, FFg - up to FFFFg, then Cannon grade and Blasting grade.)
Expansion Ratio: In a single “system” the ratio of the total of the initial chamber (unfired caseless bullet inserted in case) volume to the entire volume of initial chamber AND volume of the bore to muzzle. The attached diagram may simplify the concept.
Intuitively, a longer barrel gives more velocity to the bullet. The expansion ratio is why. Greater expansion ratio means the expanding powder has more time to maintain pressure and push the bullet. It also gives more velocity per unit of powder mass.
Kernel: One ‘piece’ of gunpowder. Depending on the type of powder, this may be anywhere from the size of fine sand (usually ‘fast powders) to near bottle cork size for cannon use.
Grain: A unit of weight in the English, avoirdupois, Apothecary and troy system; defined as 1/7000th of a pound. It is equivalent to 64.8 (rounded off) milligrams. It is a very convenient size unit for gunpowder, bullets, cases and the like.
Gunpowder: A somewhat unstable nitrate compound. Gunpowder is a solid propellant, which contains a reasonable amount of chemical energy which can readily be converted to kinetic energy. The term ‘unstable’ in this paragraph refers to the quality of being fairly easy to ignite. Smokeless gunpowder is far more stable than gasoline, for instance.
Smokeless gunpowder burns quickly (23,000 feet per second) when confined. In normal atmospheric pressure, smokeless powder is rather disappointing when ignited.
Just for the record, black powder burns at nearly the same rate either confined or open. Black powder will blow tree stumps out of the ground (very important in the days before tractors and backhoes), smokeless powder cannot in the same manner. Black powder can be quite treacherous if mishandled.
Wikipedia has a pretty good exposition on the subject. I will not repeat it here.
Wood also burns, but one notes gunpowder is easier to ignite. One could theoretically build a cannon using wood as a propellant, but it would awkward to ignite and a lengthy time lapse between ignition and firing. Muzzle velocity would be disappointing.
Single Base Gunpowder: A chemical compound wherein the energy is derived from a single chemical source called nitrocellulose.
Double Base Gunpowder: The same as single base powder, except an additional energy source - nitroglycerine - added during the chemical compounding stages of manufacture.
Triple Base Gunpowder: Same as double base except for a third energy component - nitroguanidine - is added for even more energy. Typically used only for large cannon ammunition.
Contrary to popular belief, most all single-based gunpowder has the same energy content per unit of mass. Double and triple base powders have more energy per unit of mass than single base powders, but again, the amount of energy per unit of mass is fairly constant. “Fast” powder does not have more energy than “slow” powder, it just releases the energy faster.
Powder to bullet mass ratio: The weight (or mass) of gunpowder compared to the weight (or mass) of the bullet. (“Weight” must be compared to weight, and mass to mass.) Usually, the higher the ratio, that is the more powder mass per bullet mass, will deliver more velocity. Intuitively, one understands more power makes the bullet go faster. Upon a moment’s reflection, one also intuitively understands there are limits to this; too much powder blows up the firearm. However, reading the reloading manual shows one can use more of a slower powder to make the bullet go faster with a degree of safety
Pressure: The continuous physical force exerted on or against an object by something in contact with it. (From Oxford Dictionaries, via Bing)
In the case of a firearm, the physical force is the expanding gases evolved from the burning of the propellant (powder). One should note the ‘pressure’ to propel the projectile from the arm is the same ‘pressure’ which operates the action (in such weapons) and the same ‘pressure’ which blows up the arm, however in different levels.
Sectional Density: The ratio of bullet weight to bullet cross-sectional area. This is determined by dividing bullet mass by the cross-section area of the bullet. Usually, one can look this up rather than do the math if it becomes important. By the way, this number indicates “mass per area”. Units of measure can be ignored, as it is only important in comparison.
- All references to “slower” and “faster” powders are relative only. As yet, there is no definitive and predictive system or chart showing the exact difference in burning rate of any two powders. Being perfectly honest, the loading manuals already show the ‘best’ selection of gunpowder for a particular cartridge with a particular bullet. Most of the time, one merely has to select the powder giving what one thinks is the best results for one’s application (high velocity, accuracy, long-term usefulness and sometimes what powder is available right now).
- Higher bullet weight (actually sectional density) demands “slower” powder. Put another way, the harder it is to get the bullet moving (greater resistance to motion), the “slower” burn rate is more efficient.
- The higher expansion ratio of the arm demands a “faster” powder.
- The higher the expansion ratio of the arm, the faster a given bullet will depart the muzzle (all other factors being the same). Simply put, longer barrels shoot faster than shorter barrels (normally).
- The greater amount of powder used - in a given cartridge - a “slower” powder is indicated.
- The greater amount of powder used - in a given cartridge - the faster the bullet (generally).
- Gas operated arms require certain pressure conditions, which restrict powder “speed”. For instance, the M1 Garand is gas operated. The gas to operate the action is bled off through a small port near the muzzle. If the gas at the bleed off point is too high, the operating rod can be bent and other parts damaged. While a “slower” powder will give a higher muzzle velocity, a “faster” powder is required to allow the pressure to abate to a safe level by the time the bullet passes the bleed off point.
- Desired specific performance criteria may require a powder “faster” than usual. For instance, competitors in NRA “Bullseye” competition typically load very fast powder and lighter bullets in the .45 ACP to reduce velocity and recoil, while still providing sufficient movement to the slide for proper functioning.
- Barrel length has a marked effect on bullet velocity, but no bearing at all on the proper speed powder used.
- Specific arms or cartridges may limit the internal pressure of the loaded cartridge. Pre-’98 Mausers are limited in pressure levels. Top break revolvers come to mind. Smith and Wesson revolvers prior to numbered models are not rated for “+p” loads. Arms originally intended for black powder cartridges. In fact, all arms have some limitation on pressure level. (Some are higher than others, obviously.)
- All gunpowders, regardless of burn rate, burn faster at a higher pressure than at lower pressure. Which implies -
- All gunpowders burn most completely and uniformly in the middle to upper end of their respective safe burning ranges.
- Pressure and Temperature have a direct correlation. When the temperature goes up, either from weather and climate or from the energy given off by the burning powder, the pressure goes up. When the pressure due to expanding gases inside the firearm goes up, the temperature goes up.
In any reloader’s life, one must choose among a variety of gunpowder to load cartridges. There is a myriad of varieties of gunpowder from which to choose.
One notes three basic forms of gunpowder:
1. “flake” (or flat), exemplified by Unique, and the ‘dot’ series of powders,
2. “extruded” which always makes me think of mechanical pencil lead broken into short bits, exemplified by the IMR (Improved Military Rifle) powders, and
3. “ball” or “spherical” powders, which are the same shape and named differently for patent or marketing reasons.
The only discernible difference is the ball or spherically shaped powders seem to meter better in powder measures. The smaller the ‘lump’, the better it meters. However, the shape has some, but not overwhelming effect on burning rate with modern coated powders.
There are numerous manufacturers of gunpowder, but the main difference between this powder and that is burning rate. Contrary to popular belief, all single-based powders have roughly the same energy content per weight as any other; the double base powders have slightly more, but all double-based powders are very similar in energy content. The burning rate simply refers to the rapidity with which the energy is delivered into the system.
For those old enough to remember ‘real’ cars, powder burn rate is similar to octane ratings for gasoline. Slower burning gasoline was used for higher compression ratio engines.
Most reloaders are familiar with the various lists of relative burning rates of powders. Typically, the list begins with the fastest powders and ends with the slowest. Please note: the lists are merely top to bottom listings; there is no reliable change in burn speed between one powder and the next. The difference between two ‘adjacent’ powders maybe 2% or might be 20%.
For anyone not familiar with the relative lists, they are available online. I found a few which are listed in no particular order.
One notes each listing has a warning roughly reading: Listed burn rates are approximate. Numerous variables preclude the possibility of exact burn rate comparisons. Relative burn rates vary based on various factors between ammunition components, caliber involved and firearm used. Perhaps based on the direction of wind and deflection from true North.
General Burning Characteristics
The pressure curve tends to explain how gunpowder burns in the firearm cartridge system. There are two pressure measurements useful in interior ballistics. The most commonly known is “Peak Pressure”. This is the pressure that may not be safely exceeded. This is the pressure level that will damage or destroy the firearm involved. The lesser-known but more commonly used is the ‘area under the curve’ pressure that actually pushes the bullet down the bore.
Pressure has several effects on the operation of a firearm. Pressure -
1. propels the projectile down the bore,
2. seals the breech end of the bore by expanding or swelling the cartridge case,
3. operates the firearm (in some cases),
4. affects accuracy by vibrating the firearm, and
5. wears out, bends or breaks moving parts and potentially blows up the firearm.
In a firearms system, there are several different pressure ‘levels’ to be considered. Please note these ‘levels’ are not exact and well-defined but are ranges of pressure; also note they overlap and are not distinct.
Seemingly the lowest level of pressure pushes the bullet down the bore and out the muzzle. I say ‘seemingly’ as one notes a bullet goes down the barrel without the brass case being expanded to seal the bore – one notes the smudges of burnt powder on the sides of the cases. Bullets can easily be expelled without the action – either recoil or gas – operating. I’ve experienced several cases where the bullet left the barrel but would not operate the action of recoil or gas operated firearms. I’ve never experienced the instance of a bullet being stuck in the barrel and operating the action. I’ve heard reports of this happening but never seen it in real life. Yes, I’ve had bullets stuck in manually operated actions that would fire again – revolvers, lever action rifles and so forth – but this is a distinct different occurrence.
The second and third levels of pressure are brass expansion and the cycling of the action. My testing has not yet disclosed which is the lower level. I suspect the reality may be dependent on the exact brass in the event, the type of firearm and action (recoil or gas operation) and the state of the springs in the firearm. The good news is the distinction really isn’t so important in the grand scale of things.
Next up the pressure continuum is the lower end of damaging pressure. Guns do not blow up at this point, but structural parts begin to be affected, damaged over time. Operating rods get bent, locking lugs are strained and perhaps cracked, top straps are stretched, cylinders are bulged, cases rupture, primers blow out and so forth.
The highest pressure levels in the continuum result in catastrophic failure; bolts are ejected out the back of receivers, cylinders, chambers, and receivers explode. Obviously, one wishes to avoid this level. No one in their right mind wants the previous level (harm) to occur, either.
Some of these factors are in conflict. One desires as much pressure to propel the bullet as possible, yet one does not desire pressure that will damage the firearm. For some purposes, for instance, precision pistol shooting, one desires a mild velocity for inherent accuracy and recoil control yet must have the pressure level required to operate the pistol.
(Note: Barrels explode only when blocked. Blockages are typically caused by a prior bullet lodging in the barrel for some reason, or outside debris (mud, snow or other) due to putting the open muzzle on the ground. One can also experience a bullet being stuck in the leade of the bore and being ‘pulled’ from the case mistakenly. If the following round will chamber, one then has the situation of firing a double weight projectile.
Also, note the ‘obstruction in the barrel’ type of damage is due to the air between the obstruction and the moving bullet being compressed and heated. Depending on the intensity of the loaded round, the bore may split like a banana peel or - if lucky - just bulge the barrel.
Overpressure cartridges blow up at the chamber. Obstructions usually blow up down the barrel.)
From the SAAMI website:
SAAMI proof pressure loads for handgun cartridges:
Pressure level (PSI) Proof charge level above normal maximum
15,000 or less 140 to 155 percent
15,100 to 18,000 135 to 150 percent
18,100 to 21,000 130 to 145 percent
21,100 and above 130 to 140 percent
SAAMI proof pressure loads for rifle cartridges:
Minimum proof pressure is 130 percent above normal maximum, rounded up to next 500.
Maximum proof pressure is 140 percent above normal maximum, rounded down to next 500.
These are NOT intended as guidelines for ‘what one may do and survive’; these are the testing loads for production weapons. These pressure levels are in the range of ‘damaging’ according to my definitions. They are intended to demonstrate the weapon in question will survive an overload without catastrophic failure.
Temperature and Effects
It should be noted that pressure and burning temperature are related; the higher the pressure, the higher the temperature. And the reverse is also true, the higher the temperature, the higher the pressure. That’s one reason not to develop ‘maximum’ loads in the winter and then shoot them in high summer.
Barrels do not wear out – in the actual process of shooting - from rubbing lead or brass on steel. The temperature of the burning powder blasts away steel from the interior of the barrel, much as a cutting torch cuts steel. The cutting as described is normally called ‘throat erosion’ and is limited to the area of the bore immediately forward of the chamber (and brass) mouth. This is where the rifling begins.
Therefore, the lower the pressure in the bore, the lower the temperature and the less throat erosion will occur.
The primary consideration for choice of gunpowder is the intended use. What does the reloader want to accomplish?
For nearly all purposes, reliability and long-term life of arm are desired. Whether shooting intruders bent on evil, enemy combatants, large dangerous game animals, deer, groundhogs, metal gongs, paper or the odd metal can, one wants all the rounds to go off on schedule and the arm to survive. Not to mention the shooter.
In all purposes, some degree of accuracy is desired. A bench-rest shooter requires a greater level of accuracy than a high-power rifle shooter. A general game hunter - usually - does not require as much accuracy as a ‘varmint’ shooter. A handgun shooter seeking deer has looser accuracy limits than a bullseye target competitor.
Power levels differ. Putting a hole in a paper target does not require the same level of energy as hunting bighorn sheep at extended ranges. Rapid fire handgun shooting is assisted by low recoil levels. Some ‘target’ disciplines dictate the power level required. For general shooting purposes, few of us desire abuse from recoil.
All the above factors require some degree of compromise for any specific purpose. This compromise is often facilitated by choice of powder.
Clean Burning Powder
One often hears the statement “[type A] is a dirty powder” or “[type B] is clean burning”. Powder burns dirtiest (least complete) when at the lowest pressure; conversely, it burns cleanest (most complete) at the highest pressure.
Try this: put a little of what everyone says is clean burning powder in an ashtray or another safe place. Light it with a match. You’ll note three things; it’s harder to light than you thought, it burns with a fair amount of smoke, and it leaves a residue. That’s an extreme example, but the lower the pressure, the less fully consumed is the propellant. Just to clarify, no powder charge ever burns totally. Cannon’s are known to leave copious amounts of unburnt powder forward of the muzzle.
Typically, when a powder ‘burns dirty’, the charge is at the lower levels of pressure. This can mean either a minimal load or a powder too slow to be effective in the cartridge. One usually finds a fair to the high spread of velocities.