This post is gonna show my level of experience with firearms (or lack there of) and potentially my age but it's something that's nagged me for a while and I'm just now thinking to ask. What exactly is twist rate? I know a lot of modern ARs have a 1:7 twist rate (and I've heard the old M16s have/had a 1:12) but what do these numbers mean? What does the 1 and 7 represent in the 1:7 twist rate? I understand that it has to do with the gyroscopic trajectory of the bullets as they rotate through the barrel and spin like a football. I've also heard/read how certain bullets can be hindered by certain twist rates and such but it's all still new to me, as it's something I've never really studied only heard of. Would greatly appreciate the information, always looking to expand my knowledge of firearms.
In the U.S., the twist rate is usually measured as 1 turn for a number of inches. The first number will almost always be a '1', and the second number will be the number of inches the bullet travels in the barrel to make one rotation. So, as troy said, a 1:7 twist rate means the bullet will rotate once for each 7 inches of barrel length; a 1:14 will be twice as slow - 1 rotation in 14 inches. The bullet will keep rotating at that rate as it travels to the target even though the velocity will be slowed by air friction. Another question often asked is how do you determine the twist rate when you don't know what it is? I usually use the old cleaning rod method. Place a tight fitting patch in the cleaning jag and start it in the barrel. Using tape, add a small "flag" to the cleaning rod near the handle. Push the patch in the barrel and measure how many inches the flag goes as it rotates one time. If the barrel is too short for the flag to make one revolution (it will be for many handguns), just do the math for half a rotation, a quarter of a rotation, etc. (Multiply the inches by two if you can get half a rotation, etc.)
Mal - please elaborate. I vaguely remember that the twist rate affects bullets differently based on their length and weight, so correct me if I'm wrong. Slower twist rates (1:10, 1:12, etc.) are better for longer and heavier bullets while rates less than 1:10 are better for smaller, lighter bullets. Is that correct?
If the bullet is spinning too slowly it isn't stable in flight and will be less accurate than if it is spinning at the ideal speed. Generally speaking LONGER bullets within the same caliber require a faster spin rate to be accurate. Most people generalize and refer to bullet weight, but that isn't always accurate. For example 243 was initially sold with a 1:10 twist. Meaning the bullet rotated one full turn every 10". That worked great with lighter bullet weights and even hunting bullets as heavy as 100 gr. But when guys started shooting 100 gr target bullets they found accuracy was not optimal. Most target bullets are more aerodynamic and tend to be longer for a given bullet weight. Today most 243's are twisted to 1:9 or some even 1:8 to handle the longer bullets. Generally speaking it is better to err on the side of a twist rate too fast rather than too slow. Shorter, lighter bullets still do OK in an 8 or 9 twist designed for much longer, heavier bullets. But the heavy bullets generally perform poorly if shot through a barrel twisted too slow. The original 223 load was 55 gr and it worked fine in a 1:12 twist. But the military has moved toward heavier and longer bullets. A 1:7 twist is standard for military rifles. Everyone has an opinion, there is very little consensus, but for my use I like a 1:8 twist on my AR's. The 7 twists do great with 55 gr up to 90 gr bullets, but probably isn't ideal with the lighter 40-50 gr bullets. The 1:8 twist works well with bullets 50 gr and less and still works up to at least 75-80 gr. For the bullet weights I shoot an 8 twist covers more bases that I'll actually shoot than the 7 twist.
Thanks for the correction. Oddly enough, I have a Reminton 700 in .243 that I bought used in the late 80s. I do not know what its twist rate is. When tested with 100 gr. factory ammo (Rem., Fed., & Win.), hits were all over the paper at 200 yds. That's when I got into reloading. I subsequently reloaded 60 & 75 gr. HPs (Sierra) and 100 gr. spitzer BTs, all using Accurate 2230 powder. After sighting in the 100 gr. rounds for a 200 yd. zero, the 60 & 75 gr. bullets were only off by about 1".
Everyone above hits the twist rate thing! For simple guidance on twist and caliber, I go to Shilen’s. @jmr40 gives the better answer on selecting twist, as more needs to be considered than just weight. http://www.shilen.com/calibersAndTwists.html
https://www.ssusa.org/articles/2019...-rifling-twist-rates-for-stabilizing-bullets/ How heavy or light a bullet will your rifle accurately shoot? In 1879, British mathematician Alfred Greenhill developed a simple formula for determining rifling twist rates for stabilizing bullets. Though overtaken somewhat by updated math and software programs, the Greenhill Formula remains an easy pencil and paper tool today.
Easy way to think of twist and twist rates... Throw a football with no spin. It will flip and flop. Throw with inadequate spin and you have the proverbial wounded duck Throw with a hard spin and you get a tight spiral
Generally speaking, I agree with you. At least, with jacketed bullets. At the risk of 'twisting off' (heh heh...), when one gets in to shooting lead, the rules change. When shooting cast bullets at higher velocities (over roughly 2200 fps, depending on twist) the bullet can fly apart. The RPMs get too high for the bullet to stay together. The bullet will simply explode. In this case, a slower twist is required. But, probably 99% of reloaders (and 90% of casters) won't get up to those speeds.
To further complicate the matter: (Like it isn't mysterious enough!) Most of the above is correct. Twist is measured by one complete rotation of the bullet while traveling so many inches forward. This is determined by the 'twist rate' built into the barrel. It is usually shown as one (1) full revolution in (xx) inches, eg. 1:7 means 1 revolution (turn) in 7 inches of travel. Like gauge, the larger the second or travel number, the slower the twist rate. In 'metric' terminology one revolution is still identified as "1". But the distance is given as (usually) millimeters. So in Europe one finds the notation "1:118" meaning "one revolution for one hundred seventeen millimeters" (which is the same as 1:7 in Imperial notation). A longer bullet requires MORE spin or twist than a shorter bullet. In the old days when the initial work was done, all bullets were made from lead (with a thin brass jacket) so the only way to make a bullet heavier was to use more lead; so the bullets were heavier. However, one finds the LENGTH of the bullet means much more than the WEIGHT of the bullet. With new technology making bullets of copper or depleted uranium, 'weight' is no longer the determining factor. "Length" is still the measure. The twist rate can only be changed by changing the barrel. If one decides the twist rate is not 'right', one cannot take a rifle or handgun to the gunsmith and have the barrel adjusted. Consequently, an improper twist rate can only be 'fixed' by a new barrel. Or by the understanding that one's barrel will simply not be accurate with bullets that are 'too heavy' (too long) or 'too light' (too short). Couple of other considerations: Twist rate is not 'point specific', it is more a 'range'. Usually the twist rate given by the many different applications available on line or by calculation gives the slowest spin needed for a given bullet. Not the only spin rate. A larger amount of 'over spin' is safer for accuracy than a small amount of 'under spin'. Yes, spinning a bullet too fast can cause stability problems, but usually one has to work hard to get overspin. The late Parker O. Ackley once built a rifle (.219 Donaldson Wasp as I recall) with a 1 in 5 twist. It spun bullets fast enough that some of the lighter constructed bullets would tear apart in flight due to the spin. But he makes no note of 'inaccuracy'. Rifles built in the early part of the 20th Century (the 1894 and 1896 Swedish Mausers and other 6.5mm calibers of that same period) used 160 grain or 10.1 gram bullets as standard. Twist rates were usually 1:7. Lighter (shorter) bullets in 120 and even 87 grain are not over spun. The .30-40 Krag-Jorgensen fired a 220 grain bullet with a 1:10 twist for comparison. Subsequent work shows the early Swedes handle 120 grain bullets quite well. U. S. rifles in 30 caliber all use 1:10 twist (except the .30 Carbine, which seems to shoot just as well with the faster twist). Those rifles being in .30 Government (.30-40 Krag), .30-03 Springfield, .30-06 Springfield and 7.62x51mm NATO (.308 Winchester) and handle lighter - shorter bullets just as well. Usually, the choice is already made by the manufacturer. Not something to keep one up at night.
blitz bullets in the 220 swift will also do this mid-flight. looks like a puff of smoke half way to the 100 yard butt. murf
The topic of RPM's for a bullet can be a little mind-blowing when you first hear how fast they spin. We're used to hearing the spin of things like electric motors, and auto engines, etc. Those things are almost standing still when compared to the spin of a bullet. Most auto engines redline in a range from 5,000 to 7,000 RPM's. For example, my 22-250 with a twist rate of 1:14 will send a 50 gr. bullet down range at about 3,700 FPS. If you do the math, that turns out to be about 190,000 RPM's. If the bullet isn't well balanced and solidly bonded, you can see why it will fly apart in flight. ["The math" for twist rates in inches is: FPS x 12 inches/foot ÷ twist rate (inches/rotation) x 60 sec/minute = RPM.]
I've always thought about it by remembering the wooden tops we used to play with as kids. When we'd first throw a top down, it was spinning real fast, and it was stable. But as the top's spin slowed, it would start to wobble, and would soon tip over. The other thing I've remembered about spin (twist) rate is wooden tops are short and big around. They're quite stable if they're spinning fast enough. But I can't imagine how fast a 1" wooden dowel that's 3' long would have to be spun to be stable.
That is among the simplest and most easily understood explanations of inadequate stabilization that I've ever read. I commend you.
It can't. A projectile with a length to diameter ratio greater than 5:1 can't be spin stabilized. Notice that tank AP is fin stabilized.
I have a Carcano M91/TS which has what they called, if I remember correctly, "progressive twist rate". It starts out "loose" and then progressively gets "tight" toward the muzzle. You can actually see this when you look down the barrel. When you think about it, it actually makes sense. Vary the twist rate accordingly to the velocity of the bullet through the barrel. But ... so much for easy formulas of bullet weight versus twist rate.
I've usually heard it referred to as "gain twist" rifling. The S&W .460 X Frame uses the same concept. The Greenhill formula still applies, rotation will be based on the final twist rate before the bullet exits the muzzle.
Longer and skinnier projectiles are more difficult to spin stabilize but its not a hard limit at a given value. Material density and mass distribution can make a difference. Many of the new bred of VLD bullets are approaching length to diameter ratios of nearly 6: 1.
The first time I did the RPM calculation I thought my calculator was broken. I found that with some real-world combinations of rifling and velocities, it wasn't hard to approach a quarter million RPM. An interesting point about length-to diameter ratios greater than 5:1 as being inherently unstable at any RPM. I must examine that a little. I never trusted gain twisting, although I know it's used in several arms. I figured the additional bullet distortion due to the increasing angle of the rifling toward the muzzle would offset any advantage gain twisting would have, except for projectiles with driving bands. But gain twist is used, so I guess I'm off base on that one. (Bullet distortion due to engraving is one of the reasons I'm thinking odd numbers of rifling lands and grooves (as opposed to even numbers) might be less harsh in terms of squooshing a bullet into the rifling. But I don't have the research facilities to test this.) Terry, 230RN A SIDE NOTE: Abstract formulas aside, an easy way to conceptualize RPM versus twist and velocity is to figure a 1000 foot per second projectile coming out of a barrel with a one in twelve inch (one foot) twist. So that's 1000 revolutions per second. Times 60 equals 60,000 turns per minute (RPM). And two thousand feet per second doubles that to 120,000 RPM. And a twist of one turn in 6 inches (half a foot) doubles that again, to 240,000 revolutions per minute.. If my arithmetic is correct and your calculator isn't broken.
But a bullet is in flight for a lot less than a minute. 12" twist = bullet is turning once every foot of travel, all the way to the target. 300 revolutions to a hundred yard target. Spin does not decay nearly as much as forward velocity.
But that 300 revolutions occurs in what period of time? Revolutions ÷ time in minutes = RPM And if it only takes (let's say) 1/10 of a second to reach 100 yards, then: 300 revolutions ÷ (60 ÷ 1/10) = 180,000 RPM And at 200 yards (neglecting slow-down) it would take 2/10 of a second and spin 600 times, so: 600 revolutions ÷ (60 ÷ 2/10) = 180,000 RPM. If my arithmetic is correct. Don't confuse RATE of spin with TOTAL NUMBER of spins. Terry, 230RPM
I don't. But don't get carried away with a fascination with big numbers. Just as well measure your next vacation in inches.
Science marches on. A guideline from the previous century obsolete. Talk about obsolete, the Greenhill formula is a very rough approximation for high velocity spitzers. Better estimates are available.