If you use 45 caliber pistols you have 1/16 for 45 colt, 1/24 for the 454 casull, and 1/20 for the 460 S&W. The 45 will handle bullets over 300gr in a Ruger,the 454 will handle bullets of 390gr, and so will the 460. How is the twist rate determined?
Twist rates
1 - 20 of 25 Posts
Joined
·
18,735 Posts
dhom, Rifling twist rates are set by the manufacturer for the standard factory load. For a 45 Colt it would be a 255gr bullet @ 860 fps. A combination of twist rate and velocity will determine the bullet's spin rate, which in turn will determine bullet stability down range. The faster a bullet is driven, the lower the twist rate needs to be to maintain down range stability. Likewise, the heavier the bullet, the higher the twist rate must be.
Just because a cartridge is capable of being loaded with a heavier bullet, it doesn't mean the bullet will stay stabilized down range. That said, handgun twist rates aren't near as "fussy" as rifles for two reasons. First, with rifles you expect the bullet to remain stable for 300 yards or more, however with a handgun, the typical shooting distance is much shorter ... more like 25~50 yards .... maybe 100 yards absolute maximum. Second, handgun bullets (especially 45 cal) are much larger in diameter than typical rifle bullets so they have way more gyro effect than a skinny little rifle bullet and will retain spin much better. As such, they will maintain stability with a much slower twist rate. A good example is a 38 Special and a 357 Mag. Both use the same diameter bullets and the same bullet weights yet the 357 Mag will develop much higher velocities. Nearly all 38/357 revolvers have a twist rate of 1:18.75, which will maintain bullet stability for nearly all .357 diameter bullets to at least 50 yards.
Just because a cartridge is capable of being loaded with a heavier bullet, it doesn't mean the bullet will stay stabilized down range. That said, handgun twist rates aren't near as "fussy" as rifles for two reasons. First, with rifles you expect the bullet to remain stable for 300 yards or more, however with a handgun, the typical shooting distance is much shorter ... more like 25~50 yards .... maybe 100 yards absolute maximum. Second, handgun bullets (especially 45 cal) are much larger in diameter than typical rifle bullets so they have way more gyro effect than a skinny little rifle bullet and will retain spin much better. As such, they will maintain stability with a much slower twist rate. A good example is a 38 Special and a 357 Mag. Both use the same diameter bullets and the same bullet weights yet the 357 Mag will develop much higher velocities. Nearly all 38/357 revolvers have a twist rate of 1:18.75, which will maintain bullet stability for nearly all .357 diameter bullets to at least 50 yards.
Joined
·
7,588 Posts
Always amazed me that bullets are at all stable in a handgun, especially a snub, with the bullet only making a small fraction of a turn.
Joined
·
408 Posts
What amazes me is the theoretical rotation rate. If I've done the math correctly, that 38/357 mentioned above with a 1 in 18.75 twist spins at over 30,000 rpm at a muzzle velocity of 800 fps. Worse, a 223 with a muzzle velocity of 3000 fps from a 1 in 9 twist barrel spins at 240,000 rpm.
Somehow I can accept the muzzle velocities, but the spin rate just seems insane lol.
Joined
·
334 Posts
200,000 RPM for a high power center fire rifle is typical.
Many years ago, I read a magazine review on a .45/70 derringer. The author showed a target at I believe seven yards with an obvious keyhole. He commented that a .45/70 round for self defense going through side ways wasn't necessarily a bad thing.
Joined
·
18,735 Posts
Twoboxer, Yes, it's amazing how fast bullets spin. The formula is: Spin rate= 12 divided by twist rate, times velocity, times 60. In your 38/357 example: 12/18.75=.64x800=512x60=30,720 rpm.
To put it in better perspective, it takes .19 seconds for the above 38 bullet to travel 25 yards. By the time the bullet has traveled 25 yards, it only makes about (.19x30,720=5837/60=97.3) 97 rotations from the muzzle to the target. This number is a little easier to digest.
Bullet spin rate slows down at the same rate as velocity so the above isn't perfectly accurate but it is close enough to get an idea. At some point down range, the bullet's spin rate will decay enough where it is no longer stable. It will begin to wobble then start tumbling. This is evidenced by an oval or "keyhole" shaped hole in a paper target.
To put it in better perspective, it takes .19 seconds for the above 38 bullet to travel 25 yards. By the time the bullet has traveled 25 yards, it only makes about (.19x30,720=5837/60=97.3) 97 rotations from the muzzle to the target. This number is a little easier to digest.
Bullet spin rate slows down at the same rate as velocity so the above isn't perfectly accurate but it is close enough to get an idea. At some point down range, the bullet's spin rate will decay enough where it is no longer stable. It will begin to wobble then start tumbling. This is evidenced by an oval or "keyhole" shaped hole in a paper target.
Joined
·
408 Posts
Thanks, that's pretty cool and I never thought about it that way. Just like I wanted to express rotation rate in rpm rather than rps because that's a real-world measure I think I "know".
I was first amazed by this when I read [in a forum] that 22LR conversions in a high-twist AR15 can fly apart before they reach the target. Given the source I didn't know whether that is really true. So it forced me to do the rotational math. And I suspect it is true because a light, narrow bullet with too low a density for extreme rotation probably can just fly apart lol.
The 3000 fps 1-in-9 223 takes .025 seconds and 100 rotations to travel 25 yards.
So, what I've learned about bullets so far lol:
- They move quick.
- They spin fast.
- But mostly they don't take up a lot of time.
Joined
·
18,735 Posts
Twoboxer,
Yes, this can be true but it depends on the actual ammo. Most all 22 LRs have a 1:16 twist rate so when fired from a rifle, a normal high velocity 40 gr bullet will achieve a velocity of about 1250 fps. This spins the bullet at 56,250 rpm. In a 1:9 twist barrel, the same bullet will spin at 100,000 rpm, which close to the threshold where soft lead bullets can fly apart. Chances are ... a 40 gr bullet would still hold together but any lighter bullet (typical MV of 1280 fps for a 36 gr CCI HP) will likely fly apart. If you shoot a 32 gr CCI Stinger at 1650 fps, the 1:9 twist rate will spin the bullet at a whopping 132,000 rpm ... guaranteed to vaporize when it leaves the muzzle.
Joined
·
6,450 Posts
rifling twist
I've always thought that shortening a barrel would impact bullet spin; but apparently it generally doesn't ???
Joined
·
483 Posts
No, not at all. Well... it makes the rpm slightly lower since the bullet won't reach _quite_ so high a velocity with the shorter bbl. But it's essentially the same.
Another reason that 22lrs _can_ exhibit poor accuracy from std. AR-15 bbls is that the twist rate is too fast for the super-soft bullets. So, the rifling can shear/strip off the outer layer of lead. Then the bullet either won't be spinning fast enough to stabilize, or will just fly crazy because the gas can now get around it and cut it all up. I don't say this happens all the time, but it can happen. It's not at all uncommon for this to happen in a rifle chambered for a cartridge intended to shoot high-speed, high-twist jacketed bullets that you decide to shoot cast through.
Another reason that 22lrs _can_ exhibit poor accuracy from std. AR-15 bbls is that the twist rate is too fast for the super-soft bullets. So, the rifling can shear/strip off the outer layer of lead. Then the bullet either won't be spinning fast enough to stabilize, or will just fly crazy because the gas can now get around it and cut it all up. I don't say this happens all the time, but it can happen. It's not at all uncommon for this to happen in a rifle chambered for a cartridge intended to shoot high-speed, high-twist jacketed bullets that you decide to shoot cast through.
Joined
·
18,735 Posts
3482jl,
Barrel length has a dramatic effect on velocity ... especially the first 4 inches.
Here's the statistics for a 38 Special 158gr factory LRN load in a 1:18.75 twist barrel:
2" barrel, 493 fps, spin rate = 18,931 rpm
3" barrel, 643 fps, spin rate = 24,691 rpm
4" barrel, 725 fps, spin rate = 27,840 rpm
5" barrel, 785 fps, spin rate = 30,144 rpm
6" barrel, 825 fps, spin rate = 31,680 rpm
As you can see, there is a 150 fps difference between a 2" and a 3" barrel but only 40 fps difference between a 5" and 6" barrel.
It is very common to see standard velocity 158 gr bullets fired from a snub nose revolver keyhole at distances as short as 10 yards. The same cartridge fired from a 3" revolver will stay stable to about 25 yards before bullet begins to wobble and finally keyhole at 30~35 yards. With a 4" barrel and the same cartridge, bullet stability will maintain to about 50 yards. As I mentioned before, bullet spin decays at the same rate as velocity so as velocity drops with distance due to air friction, spin rate drops at the same rate, also from air friction. Once the bullet's spin rate drops below the minimum needed to support gyro effect, it will begin to wobble and a few yards further, it will start to tumble.
Why do bullets get unstable? Simple answer .. either the bullet was not made perfectly round with no voids (air bubbles) or the bullet's center of gravity is not at the center of the bullet's length. In other words, all bullets except hollow base wadcutters (HBWCs) or dual ended wadcutters (DEWCs) are base heavy so the bullet naturally wants to swap ends and put the heavier end in front. DEWCs are unique because they are the only bullets that have a perfect center of gravity. As long as they are spinning the slightest amount, they will maintain stability until they finally hit the ground. HBWCs are extremely nose heavy so you would think they would maintain perfect stability too .... but they don't. Turns out the light rear skirt doesn't like to be spun very fast and will destabilize. The faster a HBWC is spun (due to higher velocity), the sooner it will lose stability. 750 fps is about the fastest you can drive a 38 Special HBWC and maintain stability.
A bullet's center of gravity is expressed as a ratio. a DEWC would have a ratio of 1:1 whereas a hollow point bullet may be 1:1.3. For the DEWC, the center of gravity would be exactly at the center of the bullet's length, whereas with a hollow point, center of gravity would be located closer to the base. The higher the ratio, the faster a bullet must be spun to maintain stability. A HBWC has a ratio of 1.5:1, making it nose heavy so the opposite is true ... the faster the spin rate, the more unstable the bullet will be.
What MZ5 said. Let's assume a 6" barrel and a 2" barrel with the same twist rates were loaded with different ammo that produced identical velocities from both barrels. The bullet spin rate would be the same from both barrels, even though the shorter barrel would have less than 1/8 of a twist.
Barrel length has a dramatic effect on velocity ... especially the first 4 inches.
Here's the statistics for a 38 Special 158gr factory LRN load in a 1:18.75 twist barrel:
2" barrel, 493 fps, spin rate = 18,931 rpm
3" barrel, 643 fps, spin rate = 24,691 rpm
4" barrel, 725 fps, spin rate = 27,840 rpm
5" barrel, 785 fps, spin rate = 30,144 rpm
6" barrel, 825 fps, spin rate = 31,680 rpm
As you can see, there is a 150 fps difference between a 2" and a 3" barrel but only 40 fps difference between a 5" and 6" barrel.
It is very common to see standard velocity 158 gr bullets fired from a snub nose revolver keyhole at distances as short as 10 yards. The same cartridge fired from a 3" revolver will stay stable to about 25 yards before bullet begins to wobble and finally keyhole at 30~35 yards. With a 4" barrel and the same cartridge, bullet stability will maintain to about 50 yards. As I mentioned before, bullet spin decays at the same rate as velocity so as velocity drops with distance due to air friction, spin rate drops at the same rate, also from air friction. Once the bullet's spin rate drops below the minimum needed to support gyro effect, it will begin to wobble and a few yards further, it will start to tumble.
Why do bullets get unstable? Simple answer .. either the bullet was not made perfectly round with no voids (air bubbles) or the bullet's center of gravity is not at the center of the bullet's length. In other words, all bullets except hollow base wadcutters (HBWCs) or dual ended wadcutters (DEWCs) are base heavy so the bullet naturally wants to swap ends and put the heavier end in front. DEWCs are unique because they are the only bullets that have a perfect center of gravity. As long as they are spinning the slightest amount, they will maintain stability until they finally hit the ground. HBWCs are extremely nose heavy so you would think they would maintain perfect stability too .... but they don't. Turns out the light rear skirt doesn't like to be spun very fast and will destabilize. The faster a HBWC is spun (due to higher velocity), the sooner it will lose stability. 750 fps is about the fastest you can drive a 38 Special HBWC and maintain stability.
A bullet's center of gravity is expressed as a ratio. a DEWC would have a ratio of 1:1 whereas a hollow point bullet may be 1:1.3. For the DEWC, the center of gravity would be exactly at the center of the bullet's length, whereas with a hollow point, center of gravity would be located closer to the base. The higher the ratio, the faster a bullet must be spun to maintain stability. A HBWC has a ratio of 1.5:1, making it nose heavy so the opposite is true ... the faster the spin rate, the more unstable the bullet will be.
Joined
·
529 Posts
Wow - guys (and gals)!
This is a very interesting thread.
Sorry, I don't have anything to add, but it gives me stuff to think about.
I was considering getting a .22 conversion (or .22 upper) for my AR, a 20" (1 in 9).
After reading this, it seems like that might be a waste of money.
Or at least I'd have to get the heaviest .22lr ammo that I can find.
Hmmm - more stuff to ponder.
Thanks! LOL
This is a very interesting thread.
Sorry, I don't have anything to add, but it gives me stuff to think about.
I was considering getting a .22 conversion (or .22 upper) for my AR, a 20" (1 in 9).
After reading this, it seems like that might be a waste of money.
Or at least I'd have to get the heaviest .22lr ammo that I can find.
Hmmm - more stuff to ponder.
Thanks!
LOL
Joined
·
408 Posts
I have a 1-in-9 Rock River and was looking into the simple conversion. Decided several things for myself, though YMMV:
1) A dedicated upper made more sense to me than a simple conversion. The twist and other issues go away.
(Note: When I got honest with myself, I realized an extra "dedicated upper" is really just a barrel waiting for a lower
)2) There wouldn't be a lot of benefit in shooting 22LR out of my RR lower whether converted or with a dedicated upper lol. The recoil, trajectory, and range would be quite different. So what's the gain of using the same lower, or even the same upper?
3) I shoot the AR at 100 & 300 yds. If I wanted to shoot 22LR at 50-100 yds, why not buy an accurate 22LR? Or a tacticool one?
That's when I decided I really didn't want to shoot 22LR. And if/when I do, it will be in a 22LR rifle, period.
Joined
·
130 Posts
I just bought an MP15-22 instead of converting. It is completely designed for the .22LR, and I don't have to worry about my AR if I let the 15-22 sit without cleaning it.
Virtually all Ruger single action 357 mag Blackhawks have a 1:16 RH twist. The Blackhack is well known for it's long distance accuracy and ability to handle heavy loads.
Ruger® New Model Blackhawk® Blued Single-Action Revolver Models
Ruger® New Model Blackhawk® Blued Single-Action Revolver Models
1 - 20 of 25 Posts
