Lots of care and time are taken to select the appropriate firearm. A precisely crafted piece of machinery primarily designed to do one thing, send a chunk of metal downrange and high speeds with the intention of killing someone or something. There are many ways of doing this and many types of bullets designed for many different purposes. We all can gather the hearsay of what round is the best, which manufacturer has the furthers traveling, hardest-hitting or bang-for-buck ammunition. So when it’s all said and done, understanding what happens to that piece of copper, steel and lead once it leaves the barrel is crucial when we’re faced with pulling the trigger to save lives or to feed our family members.
Ideally when a bullet is fired it travels in a straight line from barrel to target. However ideal worlds do not have air resistance and gravity, the two most hampering factors when a bullet’s path are scrutinized. Many different tools have been manufactured to ensure that a bullet makes it mark every time the trigger is pulled.
When we mount a scope on our rifles, or adjust our sights we are not changing any physical properties about the direction of our barrel or bullet, instead we are using our optics system to create a constant variable of sight to compensate for gravity and resistance. So if you were to place your barrel parallel to the ground your cross hairs may fall above and to either side of your intended target. When you hold the weapon and train the cross-hairs on the target you have adjusted it so that you are actually holding the weapon at an slight angle, preferably the proper angle that will take the bullet in whatever direction you have deemed fit to place a round into a desired space at a certain range while at the same time compensating for gravity and air resistance.
When the bullet does leave a barrel, it does so at a velocity (speed) determined by:
- The bullet’s weight. (The weight measured in grains. Each grain is 1/7,000th of a pound),
- The size of the charge (gunpowder, also measured in grains),
- The length of the barrel, and
- The type of bullet. (Round nose versus pointed nose.)
The velocity determines the distance traveled before it hits the ground and the energy it puts into a target when it hits. The higher the speed (closer the target as bullets lose speed over distance and time) and heavier the bullet, then the greater the damage.
When utilizing firearms there are two distances that you must be familiar with. These are the maximum range of the weapon and the maximum effective range. The latter being the farthest distance a round can accurately be placed into a target. Maximum range is the distance the bullet will travel before hitting the ground. It is true that different types of rounds, FMJ, hollow-core, partition, etc. will extend and decrease these distances however their effect is usually not more than [a difference of] a few yards.
On a side note; rifling greatly increases accuracy over distance by spinning the bullet. This creates a gyroscopic effect that stabilizes the projectile. The extent of this twisting is displayed as a barrels ‘twist rate’ and is measured by the number inches it takes for the groves to make one complete rotation within the barrel. 1 in 10” and 1 in 6” are examples of barrel twist rates. Different barrels for different rounds have a different number of grooves all manufactured to be the ideal number and twist rate for the intended round. Lower velocity/grain bullets (.22 LR is typically 1 in 16”) require lower twist rates to achieve nominal accuracy and vice versa for heavier weight bullets (.30-06 is typically 1 in 10”). Longer barrels mean more rifling (in most cases), different thicknesses mean reduced vibration, better heat dissipation and better pressure control. Over-rifling a barrel (too tight a twist) will actually spiral the bullet in a manner earlier in flight than designed, as described in the next paragraph.
As it’s velocity slows at greater distances, this spinning begins to affect bullet by spiraling it as it still takes a straight path. So when it impacts a target at high distances this spiraling will actually continue to affect the bullet as it enters and exits the target creating a slightly curved path. Ballistic Forensics experts use this property of projectiles to determine the distance that the bullet was fired from and is an important factor when firing on targets at great distances. This is most apparent when considering ‘grouping’ at different distances as groups become larger.
So now our Winchester .30-06 150 grain soft-point is flying through the air. During it’s flight, it would have left the barrel at 2,920 feet per second (f.p.s.) the bullet would have risen approximately 3 inches at 100 yards (That compensation we included when sighting our scope) before dropping an inch (because of gravity) and slowing to about 2,600 f.p.s. (due to air resistance) on impact at 150 yards. At that distance the circular error probability from typical 1 in 10” rifling would have increased from 0” at muzzle to about 2.5” creating a target area 5”across the chest.
Our unlucky mutant-biker gets hit square in the sternum at 150 yards, just 0.2 seconds after the projectile leaves the barrel. In .0005 seconds the bullet will have entered and exited it’s target. At .0001, the bullet hits and enters a depth of about 1.5cm or the width of your pinky nail before deforming as it was designed to do. The immense energy of the impact (2,800 Joules) will open a cavity 20 cm (8 in.) wide at .0002 as the bullet fragments and transfers it’s energy into his body before exiting his back. At .0004 the energy dissipates and the cavity collapses, jettisoning the liquefied flesh, bone and muscle through an exit wound approximately 15cm (6 in.) wide starting from the back of the sternum in a spray. One can see why this is a desirable sniper cartridge.
For a more typical scenario the same target, same distance but with a 7.62×39 Steel Core AK-47 round will hit with less velocity, around 2,400 f.p.s. The bullet is not designed to fragment, and at 150 yards, it will enter and exit cleanly if not hindered (by bone, metal, or Kevlar). However the impact and entrance will create a temporary cavity 3-4 inches wide and will shock nearby vital organs and cause them to hemorrhage. It is important to note that some bullets will actually ‘tumble’ inside victims or curve heavily due to a change in resistance (.22 LR ammunition is notorious for this). This is different from the spiral effect created from rifled barrels and is more important on a medical basis than accuracy.
It is very advisable that you research your preferred round’s impact profiles before purchasing what you think is a good round. Also research real-life effectiveness of the rounds. (From actual use in the field.) News stories are rampant of cops who fired multiple 9mm rounds and failed to incapacitate a criminal who had time to fire his own weapon before succumbing to his wounds.
JWR Adds: If the foregoing seems like complex list of variables, it is actually describing just the major ones. There are others, such as temperature, elevation, air pressure, and wind velocity. Bullet designs and their various terminal effects also differ widely. There are also many other factors that must be masteredsuch as parallax, shooting positions, uphill/downhill offset, and the shooter’s control of breathing. For more details, I highly recommend the book Ultimate Sniper 