30 Cal Rifling Twist Rate

Schutzenbob · 02-23-2015, 10:41 PM

I think something got lost along the way......I thought the question was about the excessive amount of twist that was used in Luger barrels; a 30 caliber barrel with a pitch of 1-10 inches will happily stabilize a 180 grain bullet.

As an example I plugged the specs from a Lyman/Ideal #311227 lead bullet into the Greenhill Formula to determine the twist necessary to stabilize such a bullet; Diameter .312, Length .538, Approximate velocity 1,200, Material lead/tin. What I got was a pitch of 1 turn in 23 inches. Now.....since luger barrels have about twice the necessary pitch, I have to wonder why? Possibility #1 Did DWM cut down rifle barrel blanks to make the pistol barrels? Possibility #2 DWM's rifling machines were not adjustable and the pitch was set for rifle barrels? Again ?quien sabe?

inputs
bullet length = .538
bullet diameter = .312
muzzle velocity fps = 1,200
bullet SG lead = 11.3

bullet SG values: 11.3 lead
8.9 copper
8.5 brass
7.8 steel

output = 1 turn in 23"

Notes

The classic Greenhill equation is

T' = 150 / L'

where the twist and the bullet length are in calibers. Removing bullet diameter from twist and length gives the equation often found:

T = 150 * D^2 / L

The Greenhill equation includes no term for muzzle velocity, and several sources suggest replacing the 150 with 180 for muzzle velocities over 2800 fps. Increasing muzzle velocity increases bullet spin, and spin provides the stability.

T = 3.5 * V^0.5 * D^2 / L

At 2800 fps, this equation is equivalent to using 185 in the Greenhill equation, and at 1840 fps, this equation is the same as Greenhill's.

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02-23-2015, 10:41 PM	#7
Schutzenbob User Join Date: Mar 2013 Posts: 53 Thanks: 108 Thanked 61 Times in 19 Posts	I think something got lost along the way......I thought the question was about the excessive amount of twist that was used in Luger barrels; a 30 caliber barrel with a pitch of 1-10 inches will happily stabilize a 180 grain bullet. As an example I plugged the specs from a Lyman/Ideal #311227 lead bullet into the Greenhill Formula to determine the twist necessary to stabilize such a bullet; Diameter .312, Length .538, Approximate velocity 1,200, Material lead/tin. What I got was a pitch of 1 turn in 23 inches. Now.....since luger barrels have about twice the necessary pitch, I have to wonder why? Possibility #1 Did DWM cut down rifle barrel blanks to make the pistol barrels? Possibility #2 DWM's rifling machines were not adjustable and the pitch was set for rifle barrels? Again ?quien sabe? inputs bullet length = .538 bullet diameter = .312 muzzle velocity fps = 1,200 bullet SG lead = 11.3 bullet SG values: 11.3 lead 8.9 copper 8.5 brass 7.8 steel output = 1 turn in 23" Notes The classic Greenhill equation is T' = 150 / L' where the twist and the bullet length are in calibers. Removing bullet diameter from twist and length gives the equation often found: T = 150 * D^2 / L The Greenhill equation includes no term for muzzle velocity, and several sources suggest replacing the 150 with 180 for muzzle velocities over 2800 fps. Increasing muzzle velocity increases bullet spin, and spin provides the stability. T = 3.5 * V^0.5 * D^2 / L At 2800 fps, this equation is equivalent to using 185 in the Greenhill equation, and at 1840 fps, this equation is the same as Greenhill's.