Rifling twist-rate primer: Part 1
by Tom Gaylord, a.k.a. B.B. Pelletier
Recently, we have had a number of questions about rifling twist rates that were attached to the twist-rate report. These questions are extremely important to the understanding of how bullets and pellets are stabilized, so I’m starting a tutorial on rifling twists today. I’ll keep adding sections as I see the need to explain more about the topic.
Today, I want to lay a basic foundation of what the rifling twist rate does. Blog reader Feinwerk asked if centerfire rifles (he said higher-power firearm rifles) had different twist rates than rimfire rifles, and the answer is yes. I’ll get to that, but let me start at a time when things were much simpler.
Early firearms shot multiple projectiles, similar to today’s shotguns that shoot birdshot and buckshot, but much cruder. It wasn’t long before people started experimenting with single projectiles. They found that single projectiles retained more of their initial energy than many smaller projectiles, so they did more damage when they connected with a target. The problem was getting them to connect.
After much experimentation, people discovered that spherical projectiles were the best for firearms. They flew the straightest because they didn’t have the irregular surfaces that created low-pressure zones to guide the bullet astray.
Then, rifling was discovered. Straight rifling (straight lands running parallel to the axis of the bore) was first used as a means of holding all the unburned gunpowder residue, of which there was much. That allowed the gun to be fired more times before cleaning. And, at some point, someone cut the grooves in a spiral — to make them longer to hold even more residue? We’ll never know for sure.
Once people saw how much straighter a spinning ball flew compared to one that was not spun intentionally, the race was on. For hundreds of years, the spinning round ball was the only bullet that was known. It reached its zenith as the patched ball used in the American rifle we know as the Kentucky — where the rifling doesn’t even engrave the lead ball, but spins it by spinning a cloth or leather patch that holds the ball tight while it’s inside the barrel. The ball gets very little distortion from the barrel — although there is a pattern around its circumference where the rifling pressed against the patched ball.
When the patched ball exits the muzzle, the patch falls away and only the bullet travels on to the target. Accuracy increased with this system, and the loading time dropped because the shooter didn’t have to engrave the lead bullet with the rifling when he loaded the bullet/ball.
The conical bullet
Things never stand still, though, and after the patched ball came into general use shooters began experimenting with bullets that were not balls, but rather longer cylinders of lead. These were the first conical bullets.
Although the round ball is very close to the same diameter as the conical bullet, it takes a lot more spin to stabilize the longer, heavier conical bullet.
A ball doesn’t need to spin very fast to be stable because its surface is smooth and regular. A conical bullet, on the other hand, is irregular — being longer than it is wide. Instead of a ball, it’s more like a spinning top that can balance only on its point as long as it spins fast enough. The longer the bullet, the faster it has to spin to remain pointed forward in flight. This attitude is called stability. If the bullet isn’t spun fast enough to remain point-forward, it’ll wobble like a top slowing down; and the varying air pressure that’s created will quickly cause it to tumble in flight. When that happens, the bullet will stray off its straight path.
This is when barrel makers began to be interested in the twist rates of their rifling. Prior to this time, they simply rifled the barrel with whatever twist rate their machinery supported. It’s a fact that the Hawken brothers rifled all their plains rifles with a 1:48″ twist, regardless of what caliber they happened to be.
With conicals, though, the twist rate does matter. Too slow and the bullet tumbles. Too fast and — well, less is known about what happens when the twist rate is too fast; but in my experience, you’re never able to get the same accuracy that you can when the twist rate is just right. A rifle that puts 10 shots into a half-inch with the right twist rate and bullet may put 10 of a different bullet that’s both lighter and shorter (and therefore both moving and spinning faster) into 1.5 inches.
The length of the barrel does not change the twist rate, nor its effect on the bullet — at least not directly. But a longer barrel sometimes does increase velocity. This is always true when black powder is used and can also be true when slower-burning smokeless powders are used.
A bullet that exits the bore of a barrel (of any length) with a 1:12″ twist rate and is traveling 1,200 f.p.s. is spinning at the rate of 1,200 revolutions per second (RPS). Speed that bullet up to 2,400 f.p.s. as it leaves the muzzle, and you increase the bullet’s spin to 2,400 RPS.
If a longer barrel causes an increase in muzzle velocity, it also causes an increase in the rotation rate of the bullet once it leaves the barrel. It does not change the barrel’s twist rate; but because the bullet is going faster; it’s also spinning faster. Reloaders take that into account when they load their cartridges. It’s possible to drive certain bullets too fast or too slow, resulting in less accuracy. Reloading is about finding a balance between the bullet and the velocity at which you launch it.
The most public and classic case of twist rates and their effects was the launch of the M16 rifle to the U.S. military. It addresses the specific question that Feinwerk asked. The early developers of the 5.56mm cartridge selected a twist rate of 1:14″ because the bullet was barely stable and would tumble and destroy flesh fast when it impacted a body. But they were focused only on the cartridge’s use in Vietnam — a conflict that was mostly conducted at short range and in very warm weather. The 1:14″ twist rate was too slow to stabilize the bullet properly beyond about 250 yards or in very cold weather. It worked great for a 40-grain .224-caliber bullet moving 4,000 f.p.s. from a .220 Swift, but was horrible when used with a 52-grain .224-caliber bullet moving 3,200 f.p.s from an M16.
Don’t confuse the caliber size in inches (.224″) with the name of the cartridge. Both the .220 Swift cartridge and the 5.56mm cartridge (M16) use the same .224″ bullets.
The twist rate for the M16 was increased to 1:12 inches, which worked better, but in time even that rate was discovered to be too slow to do everything the military wanted. Today, the twist rate for an M16 variant rifle runs anywhere from 1:7″ to 1:10″…depending on the specific model of gun, when it was made, which service owns it and what kind of ammunition it’s expected to shoot.
And the answer, Feinwerk, is yes…the twist rate of centerfire rifles does vary by caliber, by the bullets used and the velocities at which they’re driven.
This first twist-rate primer report was written at a very high level. I don’t know whether or not it addresses everything you wanted to know, so I’ll read your comments with interest. If we need to go into greater detail, that’s always possible. Otherwise, I’ll remain at this overview level in the next report.