Is Greenhill Wrong?

[JohnH]

All the twist rate formulas I run the Lee 429-310-RF through says it needs a 1:28 twist to stabilize. Yet my Marlin 1894 with 1:38 microgroove throws them into 2 1/2" groups with no sign of yawing whatsoever. (50 yards) I figure the twist rate formulas are Greenhill or based on Greenhill. So what gives? Or should I just not smell the gift fish :)

[mike in co]

greenhill is probably correct for what it was/is. it aint today with wheel weights and modern guns....

[Buckshot]

.................I believe tat Greenhills formula is actually based on artillery. He uses a constant that scientific folks say needs to be changed when using it for small arms.

..............Buckshot

[joeb33050]

All the twist rate formulas I run the Lee 429-310-RF through says it needs a 1:28 twist to stabilize. Yet my Marlin 1894 with 1:38 microgroove throws them into 2 1/2" groups with no sign of yawing whatsoever. (50 yards) I figure the twist rate formulas are Greenhill or based on Greenhill. So what gives? Or should I just not smell the gift fish :)
The Greenhill formula says that a 28" twist barrel will stabilize a .99" long .429" bullet. Also all shorter bullets, not any that are longer. Your Lee bullet isn't anything like an inch long.
The Greenhill formula tells you the MAXIMUM length bullet for any twist/caliber combination.
Greenhill works for any caliber up to .5, at any velocity over ~600fps, at any range up to ~500 yards.
At long range, some of the BP guys use faster-than-Greenhill twists.
At shorter ranges and higher velocities some of the 30BR jacketed guys are using twists slower than Greenhill.
Greenhill is just too simple and easy for some, hence the variations. Stability is a distant function of velocity, MV doesn't affect stability much. Bullet density and medium (air) density have a larger effect.
Greenhill Rules!
joe b.

[Bass Ackward]

Nicely put Joe!

[Junior1942]

For medium to low velocity cast loads I plug in a constant of 120 instead of 150 in the formula.

[trk]

If you go back and read the history of the formula, you will find that it was derived for a particular shape of bullet (artillary). Changing the shape will (albeit small) make a change in the results. It is one of those 'assumptions' we commonly make.

[Tom Myers]

JohnH

The Twist, Bullet Length, Caliber, Velocity calculator at this link takes into consideration the velocity that you will be using.

Precision Twist Rate Calculator (http://www.uslink.net/~tom1/twistrate.htm)

Plug in your numbers and a velocity of around 1288 fps and your twist rate shoud be close to optimum.

If, to suite your velocity application, you would rather use the Greenhill formula and substitute a constant lower than the 150 number, you can use the constant associated with these velocity ranges.

800 fps to 1000 fps, use 105
1000 fps to 1200 fps, use 116
1200 fps to 1400 fps, use 128
1400 fps to 1600 fps, use 136
1600 fps to 1800 fps, use 144
1800 fps to 2000 fps, use 153

Using the constant associated with the above velocity ranges in the Greenhill formula will return results similar to the Precision Twist Rate Calculator.

Hope this Helps.
Tom Myers
Precision Ballistics and Records (http://www.tmtpages.com/)

[Leftoverdj]

Without bothering with the theory, 2 1/2" groups at 50 yards tells me that something ain't right. I got similar results, maybe a little worse, with my mid 70s 1894 and the Lee 310. Since the same rifle will easily beat that at 100 yards with the Lyman KT 255, I gave up on the Lee quick.

[JohnH]

Don't know what I did wrong first time, but went back and plugged in .430 diameter, 1300 velocity, .85 bullet length, get 1:32.

[swheeler]

Is Greenhill Wrong? The simple answer-NO