02 January 2013

Firearm Lethality: Pressure

The most important aspect of bullet accuracy is uniformity.  The powder/primer combination that turns solids into gasses to create a pressure curve needs to be very uniform which means a uniform burn rate of the powder, and a uniform resistance of the bullet to the bore.  With the same type of bullet it is not uncommon at all to get different accuracy from different powders.  This is due to harmonic vibrations and barrel timing, as well as pressure curve differences during bore time.  Bore time is the term for how long the bullet spends in the rifle after the primer has detonated.  I'm not going to go into great detail about internal ballistics, just touch on the most common types of propellants; black powder, flake powder, ball powder, and stick powder.

M1A destroyed by old '59 German milsurp 7.62x51
Primers are a "primary explosive" which means they will detonate on their own.  Smokeless powder is also an explosive, but in firearms we aren't going for an "explosion" or "detonation."  Black powder explodes, it does not burn.  We are looking to use a primary explosive to cause "deflagration" of smokeless powder, basically causing it to burn, while some sort of spark/fire (from a match, fuse, or primer) will cause black powder to explode.

Soldiers in various wars may have used C4 explosives to boil water in the field.  C4 burns very hot, and as long as you let it burn out it won't explode, however, the moment when pressure is combined with that burning heat, someone is losing a limb.  Smokeless powder will burn at atmospheric pressure quickly, so you can't use it like C4 to boil water.  But just like C4, the right combination of heat and pressure will cause it to detonate.  Safe loadings are designed to keep smokeless powder from exploding.

Why you want powder to burn, not detonate
Thankfully smokeless powder isn't as sensitive as C4, but if you let pressure get too high too quickly inside of a cartridge case the effect is the same, a loaded cartridge becomes a bomb waiting to happen.  So the key to making safe ammo is to control the pressure, by using the right amount of a powder with the appropriate burn rate.

In the first post in this series we took a look at lead projectiles being fired by black powder.  It is not a very efficient explosive as 55% of the combustion products are solids instead of gas.  But black powder explodes faster than smokeless powder burns, so in order to harness that energy bore sizes for black powder firearms are large.  The bigger an area at the base of the bullet that gas pressure has to push against the more efficiently it can move the projectile down the bore.  When a solid turns to gas, pressure is equal on all surfaces that the gas pushes against.  So the larger the bullet base, the larger a percentage of total pressure that the bullet can receive at any given time.  This is true for air guns, black powder firearms, and smokeless powder firearms.  Black powder firearms have to have large bores or the pressure would overcome the strength of the steel and turn the firearm into a bomb.
Bulky powders with >60% case fill help
prevent double charges in pistol cases

There are different "grades" of black powder, which have different speeds of detonation based on grain size.   Smokeless powder does not come in grades, but in different names with associated uses and burn rates.  Compared to smokeless powder, black powder is very uniform.  Smokeless powder ranges from ultra fast pistol powders (Norma R1), to powders slow enough to propel 20mm cannon shells (WC872).  When the range of ballistic performance is compared, black powder seems very uniform.

Without going into the details of converting a mol of smokeless powder into the equivalent molar mass of gas (I did it once to show the process) the basic volumetric change is you can expect 2.4 liters of gas from a powder charge that goes into the average 308, but at the high temperature that the gas coming off the burning powder it will be over 7 liters in volume due to heat.  Think about how much energy is in a Ford 7.3 liter Powerstroke diesel engine.  Then imagine every cylinder firing at the exact same time.  That is about the best mental comparison I can think of for how much energy is released when a rifle cartridge is touched off, and energy wise it is a poor comparison.

Leaving black powder behind, smokeless powder comes in three main shapes and three types.  Flake, stick, and ball shapes, single, double, or triple base as far as chemistry.  Single base is nitrocellulose.  Double base is nitrocellulose plus nitroglycerin or nitroguanidine.  Triple base is all three.  The shape of the powder influences things like burn rate, ease of ignition, and potential case fill (there are some powders that are "sponge" shaped by mashing them with salts and then rinsing the salt away during the manufacturing process, but I don't know of any company still using that method). 

Unique pistol/shotgun powder
very versatile
Flake powders are generally used now with pistols and shotguns.  The flat flakes have the highest surface to volume ratio of any of the powders, are generally the easiest powders to ignite, and are generally formulated for a fast burn rate due to their intended use in pistols.  It should be noted that Turk 8x57 surplus ammunition was loaded for a number of years with a flake rifle powder, so you might still find some flake rifle powder around, but it is uncommon.  Some handloaders use pistol/shotgun powders like Unique and Red Dot for reduced power rifle loads with excellent results (anyone interested should google "Ed Harris 13gr Red Dot" for more info).

Extruded powder Left, Ball powder Right
note the denser pack of ball powder
Stick powder, or "extruded" powder is generally used with rifles.  This has the lowest surface to volume ratio of any of the powders (as measured by mass, the individual kernels are much larger than ball so the same mass of ball kernels will have much more surface area than the equivalent mass of stick), but ironically because of how it "packs" in a case (more empty space between kernels of powder) it is easier to ignite consistently than ball powder.  More accuracy records have been set with extruded powder than ball powder, and it is my educated guess that the consistency of ignition is the reason behind this.  Water content of stick powders affects burn rate, so powder factories making extruded powder must account for this.  There is a reason that IMR extruded rifle powders are made in Canada.

Stick powder from Australia,
note the golden hue of the coating
Ball powders are found for both rifle and pistol.  The method for manufacturing ball powders can be done entirely under water, which is why St. Marks powder plant in Florida specializes in ball powder, as the production staff does not have to worry about humidity the way their Canadian counterparts do.  Ball powders meter the best of any shape in volumetric powder measures, and with the exception of sniper ammunition, all ammo used by the US military is loaded with ball powder.  The common wisdom that stick powder is better than ball powder comes from lots of M118SB loaded with WC846 powder, which did not shoot as well as the lots loaded with IMR4895.  In the mid 90's with the adoption of the M134 minigun a study was done on M80 ball ammo loaded with WC846 and it was found that the arsenal primers wouldn't consistently light that powder, so a powder change to WC846FS (flash suppressed) was made.

To sum up, your primer and propellent is there to create pressure by turning chemical energy into kinetic energy.  Too little is just as bad as too much, and using the wrong burn rate is potentially disastrous.  Since Accuracy is the most important aspect of lethality, it is important to have a consistent primer/powder interface that produces a shot to shot uniform pressure curve.

At the end of the day, the proof that the primer and powder did their job correctly is a hole in a target.  Modern loaders, commercial and hobbyist alike, have an amazing assortment of propellents to choose from in the quest for accuracy.

Remember that accuracy is always relative against what you are shooting.  A benchrest rifle capable of 1/8 MOA accuracy at 200 yards is more than you need for deer.  A 4 MOA service rifle is plenty for harvesting deer out to 200 yards, as long as you can maintain that level of accuracy.  Some handloaders will find a very accurate load that is at a low velocity, and choose to push the bullet faster even though it might open groups up from .5 MOA to .9 MOA.  Giving the bullet extra energy and momentum is a good thing, as the difference of .4 MOA is largely academic in most hunting/defense situations. 

9 comments:

GreyLocke said...

Do you have the back story on the kaboom's? I'd be interested in knowing what went wrong.

D Anderson said...

The article says that stick powders have the lowest surface area to volume ratio of any powders. However, I happen to know that spheres (balls) have the lowest surface to volume ratio of any three dimensional shape. I don't mean to slam your article. I just figure you're the kind of guy who likes to get his facts right.

AM said...

D Anderson, that is a good point and you are completely correct.

However, I was referring to the powder not as an individual kernel, but as all the kernels in the brass.

As you can see the extruded kernels are larger, and take up more volume per kernel. The ball kernels are smaller and there are more of them per unit of volume. That means for the same mass of powder, the stick kernels will have less surface are (total) than the ball kernels.

So lots of little balls have a lot more surface area than a few bigger cylinders.

I should have been more specific in my wording, thank you again for pointing it out. I will update the post to point out that it is surface area per mass off powder, not per kernel.

AM said...

Greylock, the M1A kaboom was with 1959 German 7.62x51 surplus ammo, a forensic exam of the remains of the rifle by Fulton Armory put the blame on an improperly heat treated barrel, which I don't buy as a complete answer. The failure of the receiver indicates an overpressure event, even if the barrel steel had been work hardened beyond safe.

The lever rifle, Marlin 45-70, unknown ammunition failure (either too much or not enough powder).

The revolver is unknown, but likely a double charge with a handload. Even a "sedate plinking load" turns into a bomb with a double charge, hence the tag that bulky powders reduce the risk of a double charge, since if you double charge with a bulky powder it spills out of the brass.

Hope this helps.

Mark said...

Thank you sir.

Anonymous said...

Imagine my surprise when searching for a .308 load in my Hodgdon manual... and finding a recipe calling for 8 grains of Titegroup in front of a 168 grain bullet. Velocity: 1080 fps, so definitely a 'reduced' load.

Haven't- and probably won't- try it, but it is interesting that we could come up with pistol powder loads for our favorite rifle, should we desire to.

AM said...

that is a subsonic load, for low noise applications

Historian said...

What I learned from various books was that the most important single factor was initial alignment of the bullet with the barrel.

AM said...

Historian, if that were the case then muzzle loading firearms should be much more accurate than smokeless powder firearms as the projectile starts out with 100% contact perfectly centered with the rifling.

If that were also the case then the same load of ammunition would not shoot well in a broad range of firearms the way Fed GMM does. You should read Dan Newberry's Optimal Charge Weight method of load development.