Twist Rate by Josh
I’ve talked before about ammunition compatibility with certain firearms and how some guns “like” one type of ammo over another, or perform better with a specific type of ammo. My past references have been in regard to pistols and shotguns, but ammunition / rifle compatibility is especially important when working to shoot your rifles as accurately as possible.
For bullets to be stabilized as they fly through the air at high velocities, they must have spin which is imparted by the rifling in barrels. Rifling is the spiral grooves machined on the inside of the bore. The rate of twist in the rifling of a barrel is an important factor in determining which bullet will be most consistent or “accurate” out of that barrel. I have seen first- hand where group sizes looked more like someone was patterning a shotgun than shooting for groups on paper. This was a result of the wrong cartridge/bullet combo for a particular twist rate. If you’re familiar enough with a particular caliber you could probably even tell what twist rate a rifle has just by seeing how certain types of ammo perform with it. So here’s a very basic explanation of twist rates that might help shed a little light on the subject.
The twist rate of a barrel is commonly expressed in inches. So if you look at a barrel or a manufacturer’s spec sheet for a particular gun you will see something like this – twist = 1/7 or 1:7 which means there is one full twist of rifling in seven inches of barrel. 1/9 = one twist of rifling in 9 inches of barrel and so on. So an 18” barrel with a 1/9 twist will have two complete twists of rifling in the barrel.
Longer bullets require a faster rate of twist to stabilize them than shorter bullets of the same caliber. Longer bullets are typically heavier, so many shooters associate bullet weight with twist rate. For example, in a .223 a 1/9 twist rate is good for 55 grain bullets but the same twist rate will not stabilize a 75 grain bullet as well. The 55 grain bullets out of a 1/9 barrel can produce sub half inch groups where the 75’s through the same barrel would probably produce 3 or 4 inch groups. The 75 grain bullet is heavier so it would seem like the weight is the critical factor but it is actually the length of the projectile that is critical.
In most cases one would be correct in seeking heavier bullets for a faster twist rate or lighter bullets for a slower twist rate but not always. The majority of bullets used in today’s ammo are copper jacketed lead core bullets. But many companies like Barnes are producing solid copper bullets and copper is not as dense as lead. Using the .223 again as an example, a 55 grain solid copper bullet will be longer than a lead core bullet, so the same 1/9 barrel mentioned above might not stabilize the 55 grain solid copper bullet as well. It may need the same twist rate that it would take to stabilize the 75 grain lead core bullets.
The velocity at which the bullet travels through the barrel is also critical as it relates to twist rate. Let’s use the .223 again as an example and the .22-250. Most .223’s will have a twist rate of 1/7, 1/8, or 1/9, while most 22-250’s will have twist rates of 1/12 or 1/14. The bullets of both of those cartridges are the same diameter so why the different twist rate? – Velocity. The 22-250 will push the same bullet through the same size barrel at a much faster speed so it does not need as fast a rate of twist to achieve the same rpm’s needed to stabilize the bullet.
Twist rate and ammo compatibility may not be the answer to every “accuracy” problem. There may be multiple issues with a shooters gun that are not allowing him/her to shoot consistently. But trying a variety of ammunition with different bullet weights (lengths) can be the first step to eliminate that one aspect.