Bullet RPM's

[Johnw...ski]

In referance to the Greenhill formula for barrel twist, for the big dia. boolits (for me anyway) it seems that twist is on the far side of being slow.

When I calculate the ideal twist rate for a .458 cal. rifle I come up with around 1 in 30" twist. That seems way slower than anything I am familiar with.
Calculating rpm's for boolits traveling between 1500 and 2000 fps I come up with between 77,000 and 102,000 rpm using a 1 in 14" twist. Even if I push up the boolit speed to 2500 fps I come up with 128,000 rpm, still not unreasonable.
So it seems to me that my 1 in 14" twist rate is not that far off considering the
1500 to 2000 fps window I would like to use. The Greenhill formula leaves something to be desired with larger bore diameters.

Am I missing something here? What do you guys think?

The twist rate is fixed in both my .458 rifles, but going by rpm's, the twist rate doesn't seem so far off. Most 45-70's seem to use a slower twist rate, therefore their rpm's must be way down. Isn't the ideal rpm range slightly over 100,000 rpm, or is that just for jacketed boolits?

What I've got is what I've got so this is really Monday morning quarterbacking, but I would really appreciate any input.

Thanks,

John

[Junior1942]

In my considerable experience, cast bullets going over 110,000 rpm hit all over and around the target. Others say differently.

[madcaster]

Junior1942,
You must have burned a lot of powder getting there!

[joeb33050]

In referance to the Greenhill formula for barrel twist, for the big dia. boolits (for me anyway) it seems that twist is on the far side of being slow.

When I calculate the ideal twist rate for a .458 cal. rifle I come up with around 1 in 30" twist. That seems way slower than anything I am familiar with.

!!!!!!!!There's something wrong with your figuring. A .458 bullet 1.25" long, weighing ~484 grains will stabilize in a 25" twist barrel, per Greenhill.
A 14" twist .458 barrel will stabilize a 2.24" bullet weighing ~864 grains, Greenhill again.
Greenhill needs bullet length, caliber/diameter and twist-if you know 2 you can calculate the 3rd. There's no RPMs in Greenhill.
Greenhill works fine, although the BPCR guys advocate a faster-than-Greenhill twist at low velocity/long range.!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1

Calculating rpm's for boolits traveling between 1500 and 2000 fps I come up with between 77,000 and 102,000 rpm using a 1 in 14" twist. Even if I push up the boolit speed to 2500 fps I come up with 128,000 rpm, still not unreasonable.
So it seems to me that my 1 in 14" twist rate is not that far off considering the
1500 to 2000 fps window I would like to use. The Greenhill formula leaves something to be desired with larger bore diameters.

Am I missing something here? What do you guys think?

The twist rate is fixed in both my .458 rifles, but going by rpm's, the twist rate doesn't seem so far off. Most 45-70's seem to use a slower twist rate, therefore their rpm's must be way down. Isn't the ideal rpm range slightly over 100,000 rpm, or is that just for jacketed boolits?

What I've got is what I've got so this is really Monday morning quarterbacking, but I would really appreciate any input.

Thanks,

John

Above..............

[Wayne Smith]

The Greenhill formula was created for artillery shells. Yes, it's a little off for us, and there are compensatory ways to figure. I just don't remember any of the compensations off the top of my head.

[44man]

Greenhill doesn't work!
Best twist for heavy boolits in the 45-70 is 1 in 18 and even a little faster would be better. My BFR revolver is 1 in 14 and will shoot under 1" at 100 yd's and clangs steel to 500 meters. It shoots better then my BPCR.
A lot of 45-70's were made with 1 in 20 or slower. They are hard to get to shoot long range.
I would rather have a faster twist because it is easier to adjust the velocity down then to try and exceed pressures to raise the spin rate for stability.

[Johnw...ski]

In my considerable experience, cast bullets going over 110,000 rpm hit all over and around the target. Others say differently.

A lot of interesting but not surprising answers.

Junior 1942, what do you consider an ideal CB spin rate, how about PB vrs. GC?

John

[Junior1942]

A lot of interesting but not surprising answers.

Junior 1942, what do you consider an ideal CB spin rate, how about PB vrs. GC?

JohnI don't have an ideal rpm figure because there is no such thing IMHO. No comments on PB vs GC, either, except to say GC bullets can go faster so their rpm figure will be higher.

[pdawg_shooter]

Years ago, when I was trying to find the limits on pure lead paper patched bullets I built a .458 Win mag on a 1914 action with a Douglas 1:22 barrel. It will shoot 550gr PP bullets at 2170 into a inch and a half with iron sights. Some 395gr LT bullets went over 2600. Same accuracy. RPM can be tough on soft bullets. Using an alloy that test 14.5 BN In a 300RUM can be taken to well over 3300 with good accuracy.

[Tom Myers]

Charley Dell published a formula that allowed for the lower muzzle velocities that some bullets are launched at:

I used his formula to create an on-line twist-rate/bullet-length/muzzle-velocity calculator that seems to work well for just about all combinations.

The mathematical algorithms used to perform the calculations are also displayed on the web-page.

Run your numbers and see if the twist rates and bullet combinations you are using make a little more sense.

Precision Twist Rate Estimator (http://www.tmtpages.com/twistrate.htm)

Hope this helps.

Tom Myers
Precision Ballistics and Records (http://www.tmtpages.com/index.htm)

[Bass Ackward]

I have a new 625 Smith in 45AR that the RPM monster is eating my shorts at 41,000 RPMs. :grin:

Need a new T-shirt, RPM barrier @ 40K.

[Woodwrkr]

I've been under the impression the rate of twist for any particular boolit was calculated using the length of the boolit, not the diameter. Is this not correct? I've recently read something about the military using a faster twist barrel in the M-16 for the heavier .223 condom's because they are longer.

How about someone educating me on this.

[Scrounger]

Velocity is also a factor, a big one. And that is why the military made the switch. They were afraid their 12 inch twist wouldn't stabilize the bullets enough in Arctic temperatures which would reduce the velocity quite a bit.

[JSnover]

As I understand the process, the military decided to increase the weight of the projectile for better downrange performance. Enemy soldiers weren't cooperating. They did things like wear body armor and refuse to fall down after being shot.
Since the only way to increase the weight of a bullet is to make it longer, the twist rate had to be made shorter. The formulas I've seen didn't mention weight. Probably because it's already factored in, if you think about it: For a given length::diameter ratio, if you change the length or the diameter, you will also change the weight (assuming you don't reconfigure the bullet by adding a hollow base, hollow point, different core materail, etc.).
Temperature extremes might also have something to do with it, but I hadn't heard that angle until now.

[felix]

The rule: the more resistance the projectile meets outside of the barrel, the more twist is needed. Period. For example: More frontal area, more twist.....more weight, less twist.....longer projectile, more twist......more side wind, more twist......higher elevation, less twist....heavier air, more twist....more time in flight, more twist.....under water, umteen twist.....outer space, close to zero twist. ... felix

Keep in mind that velocity effectively increases the frontal area. ... felix

[JSnover]

The rule: the more resistance the projectile meets outside of the barrel, the more twist is needed. Period. For example: More frontal area, more twist.....more weight, less twist.....longer projectile, more twist......more side wind, more twist......higher elevation, less twist....heavier air, more twist....more time in flight, more twist.....under water, umteen twist.....outer space, close to zero twist. ... felix

Keep in mind that velocity effectively increases the frontal area. ... felix

Heh! All I needed was for somebody to dumb it down for me. Thanks!

[JIMinPHX]

I've been under the impression the rate of twist for any particular boolit was calculated using the length of the boolit, not the diameter. Is this not correct? I've recently read something about the military using a faster twist barrel in the M-16 for the heavier .223 condom's because they are longer.

How about someone educating me on this.

The military has gone down to 1:7 twist because of some of the extremely long late-ignition tracer rounds that they sometimes fire. You need to pick your twist for the longest projectile that you are going to use.

I did a bunch of Greenhill tests several years ago with a .223. The formula proved pretty close to correct in my experiments. I did not try it with different calibers. I did not try it with boolits that had large meplats.

Length & diameter of the boolit are both used in the Greenhill calculation. A correction needs to be made for velocity & boolit density changes. Lesser adjustments can be made for ambient temperature, humidity & altitude, although these last three are so small that I tend to ignore them.

More info here –
http://castboolits.gunloads.com/showthread.php?p=287259#post287259