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A Look At Ballistic Coefficient And Twist Rates In Choosing Ammunition

When it comes to first choosing ammunition for your new rifle it can feel a bit daunting. From different bullet weights, to bullet types and shapes, to brass or steel cased. So many options it can make one’s head swim, especially if you are a new shooter. Experimenting with multiple different ammunitions to fit your firearms needs can be frustrating, expensive, and depending on what caliber you’re shooting can be hard on your barrel with a ton of ammo experimentation. The most important factor to consider is your rifle’s configuration and purpose. Factors such as rifling twist rate and ballistic coefficient, are all important factors that can and will play a role in being successful in our first ammunition choices.

The most important factor for ammunition choice is determining how “fast” the rifling twist in your barrel is. Your rifling is what puts the spin on your bullet and allows your bullet to stabilize in flight. Think of how a football flies farther and more accurately when thrown with a perfect spiral compared to when it thrown like a “dead duck” and flutters as it flies through the air. When your bullet is properly stabilized with the optimal twist rate it allows your bullets to fly like a football thrown with a perfect spiral enhancing your rifles accuracy. A barrel’s twist rate refers to the length of barrel that it takes for your bullet to make one full rotation as it travels down the barrel and is represented as a 1 rotation in “X” amount of distance in either inches or centimeters. Most calibers have fairly common twist rates such as 1-10” for .308 Winchester, 1-9” for .223 Remington, 1-8” for 6.5 Creedmoor, and so on. However, this does not mean that all barrels chambered for a certain cartridge will have the same twist rate and it is in your best interest to verify your twist rate yourself. I’ve found the best way to determine this has been using a cleaning rod with a rotating handle and either a brush or jag. By marking the rod and measuring the distance it takes for the rod to make one full rotation as you push it down the barrel you can determine your barrels twist rate on your own. The general rule of thumb is that the “faster” your twist or shorter the distance it takes for your bullet to make one rotation, the heavier bullet you can get to stabilize out of your barrel. Matching your barrels twist rate to the correct weight ammunition can often get you a head start to shooting your firearm to its full accuracy potential.

The second and probably most technical aspect to bullet choice is ballistic coefficient. A bullet’s ballistic coefficient is a measure of how well a bullet penetrates the air in pounds per square inch. The higher the ballistic coefficient is, the more efficient a bullet is to penetrate the air. The more efficiently a bullet can fly, the less velocity we lose to air resistance or drag. Losing less velocity to air resistance equates to a bullet that is able to fly faster for a longer distance. Having a faster bullet also reduces flight time which also means less time for wind to interact and have an effect on our bullets. Ballistic coefficients are determined through a mathematical formula using the bullet’s shape, weight, and diameter and are comparative measurements for the given bullet to another similar “model” bullet. The two most common models are the standard rifle bullet (G1) and the long, sleek, boat-tailed rifle bullets used for longer range shooting (G7). Most commonly ballistic coefficients will increase the heavier a bullet gets. This is slightly counterintuitive, however if you think of throwing a golf ball (a heavy high ballistic coefficient) to a ping pong ball (light low ballistic coefficient) projectile accurately in the wind, it would be much easier to get the golf ball with its heavy weight to resist the wind and hit your target. It is important to remember though that since bullet diameter is always the same the only way to increase weight is through adding length, which will require a faster twist to get the spin required to stabilize a longer bullet.

A little experimentation with different ammo types will always be necessary for finding the ultimate performance, however using these general guidelines to zero in on what may work for you and your rifle’s needs you can avoid wasting more of your time, money, and barrel life shooting the wrong bullets. It is also important to remember that each firearm is different and what worked great for your range buddies rifle may not work for you even though you may have the same exact rifle. Also, remember these are guidelines and not meant to be hard line rules so don’t be afraid to experiment outside these “rules” if you want, variety is the spice of life, and you just might find something that shoots really great!