Internal and External Ballistics The things you can’t see. After the trigger is pulled on a firearm, a lot goes on inside and outside the gun that you can’t see. Many of these invisible things are responsible for events you don’t normally think about. Internal Ballistics It starts with the firing pin or striker hitting the primer hard enough to cause it to detonate. The primer flash has to produce enough heat for a long enough period of time to ignite the cartridge’s smokeless gunpowder. In general, the brand of primers is not critical but it is a well-known fact that some match grade loads prefer a certain brand for optimum accuracy. A light-struck primer may still ignite the powder but it will not have as much flash as a primer with a full power strike, thus the cartridge will produce sub standard results. This becomes evident when you chronograph a string of 10 rounds and record the inconsistent velocities. As soon as the primer flashes, it will be thrust to the rear, pushed partially out of the pocket until it contacts the breech face. As the first few kernels of powder ignite, chamber pressure starts building that pushes the bullet forward and pushes case rearward to reseat the primer. Within a few thousandths of a second and an inch or so of bullet travel, chamber pressure and temperature will peak, however this doesn’t mean the powder has completely burned up, it just means pressure will start dropping. Next is the burn rate of the powder. Burn rate charts will tell you the order of burn rates from fastest to slowest when powder is burned in open air but don’t tell you what burn rate is optimum for your cartridge and bullet weight combination. Most reloading manuals do a good job of listing powders that are within acceptable burn rates. Burn rates can change radically depending on the volume of the cartridge and the weight of the bullet. The easiest way to express burn rates is to determine how many inches of bullet travel it takes to totally burn up. Fast burning handgun powders such as Bullseye typically get a 100% burn in less than two inches of bullet travel. This makes fast burning powder a good candidate for low velocity target loads. Slow burning magnum powder such as W-296 will take about 15 inches of bullet travel to burn up. Because it takes so long to burn up, slow burning powder is best suited for magnum handgun velocities (1200 fps or faster) All rifle powders are considered “slow burning” and take from 16 to 26 inches of bullet travel to totally burn up. As long as powder is still burning, it will create enough pressure to accelerate a bullet. This means a slow burning powder will push a bullet for a longer period of time, thus it will generate higher velocities than a fast burning powder and it will generate lower chamber pressure in the process. A software package named QuickLOAD will predict a given load’s parameters such as chamber pressure, velocity, bullet travel time, powder burn time, and many more. Although computer generated statistics in QuickLOAD are not as accurate as actual lab tests with real ammo, it does provide a reasonably accurate picture of when and what happens inside the chamber and bore When the powder’s burn rate best matches the weight of the bullet, the capacity of the case, and the length of the barrel, it will produce the best accuracy. 1 Burning powder accelerates at a rate of about 6000 feet per second (fps). As long as powder is still burning, it will push a bullet faster and faster, until the bullet finally exits the muzzle. Meantime, the bullet goes through several transitions. This starts as the bullet first begins to exit the case. It will contact the chamber throat (or cylinder throat). Chamber pressure will cause the bullet to expand slightly, causing it to seal in the bore. This process is called “obturation”, meaning to bump up in diameter until the cylinder or barrel throat restricts further expansion. As the bullet passes through the cylinder or chamber throat, it will be swaged (sized) to the proper diameter for the bore. In revolvers, the primary forcing cone is located in the mouth of the barrel and will guide the bullet into the bore. This transition is the most efficient when chamber pressure matches the hardness of the bullet to force it to obturate and seal in the bore. For lead bullets in handguns, the formula is: Bullet hardness (BHN) = chamber pressure divided by 1400. Jacketed bullets also obturate but require considerably more pressure. Handgun jacketed bullets have a thinner and softer jacket so chamber pressures closest to the SAAMI max rated pressure for the cartridge seem to be the most accurate. Most rifle chamber pressures are in the range of 50,000 psi, which will force a thick jacketed rifle bullet to obturate. With jacketed bullets, you are at the mercy of the manufacturer to make bullets with the proper jacket thickness and hardness. As such, you may find some reduced power loads using jacketed bullet don’t work very well. A bullet’s “ogive” is the diameter where the bullet first contacts the throat. As an example, the ogive for a round nose .308 bullet would be where the diameter is about .296”. The angle of the throat may not be friendly with the angle on the bullet’s ogive. This is one of the many issues that can cause a rifle to be “ammo fussy”, where one type of bullet is far more accurate than anther type. Assuming a friendly throat contact, there will be minimum bullet damage. Bullet seating depth is very important because the bullet has already increased in velocity by the time it strikes the throat. Too much pre-throat bullet travel will cause higher velocity and excessive bullet strike damage, which will affect accuracy. Too little pre-throat bullet travel will increase chamber pressure, possibly to dangerously high levels that can damage the gun. Pre-throat bullet travel is controlled with bullet seating depth. Using the reloading manual’s COL is highly recommended. The bullet contacts the lands and grooves in the bore called “rifling”. This will engrave the bullet with a reverse image of the rifling. In other words, the lands that extend from the bore will create grooves in the bullet and the grooves in the bore will create lands on the bullet. The engraving process takes considerable starting chamber pressure …. more for jacketed bullets, less for lead bullets. Once the bullet has been engraved and is pushed down the bore by rapidly expanding gasses that create chamber pressure, a seal will be established between the bullet circumference and the bore. Because lead bullets are much softer than jacketed bullets, pressure keeps them expanded so they tend to seal much better in the bore and result in higher velocity than an equal powder charge pushing a equal weight jacketed bullet. Bullet to bore friction becomes negligible when a fractional ounce bullet is being pushed by several tons of pressure. While the bullet is transitioning through the bore, the rifling twist rate combined with the bullet’s muzzle velocity will cause to bullet to spin very rapidly to maintain downrange stability. Rifling twist rates are expressed in a ratio such as 8:1. This would mean the bullet spins one complete revolution in every 8 inches of travel. The bullet’s spin rate can be determined with the following formula: Spin rate in RPM = 12 divided by twist rate, times muzzle velocity, times 60. As an example, lets say a 223 Rem bullet exits the 2 muzzle at 3240 fps and the barrel has a 1:9 twist rate. 12/9=1.333 X 3240 = 4320 X 60 = 259,200 RPM. Bullet spin will create torque while inside the bore and will actually twist the firearm enough to be seen and felt by the shooter. A heavier bullet combined with a faster twist rate will increase barrel torque considerably. This sudden twisting of the firearm can wreak havoc with accuracy. Typical handguns have a slow twist rate for this very reason. Longer shooting distance is not an issue like it is with a rifle so the rifling twist rate in a handgun is much slower to accommodate larger diameter heavy bullets and prevent excessive barrel torque. The pressure generated by burning powder in conjunction with the mass and length of the barrel will cause the barrel to vibrate. Because of the rifling, barrel vibrations tend to make the muzzle move in a circular motion in the same direction as rifling. This phenomenon is commonly called “barrel harmonics” and only applies to rifles because handgun barrels are normally too short to be affected by harmonics. As a bullet moves closer to the muzzle, the frequency generated by the vibrations increases …. much like pulling the slide closer on a slide trombone will increase the frequency (higher pitch notes). From the true meaning of harmonics, frequencies will add algebraically to produce a very complex waveform. A modest difference in muzzle velocity will change the waveform and make the end of the muzzle rotate at a different rate. As the muzzle rotates from harmonics, it will result in bullets being launched at slightly different angles. This will show up on a target as a group of holes rather than one single hole. The less harmonic control you have, the larger the group will be.