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Caliber ...Twist
.172 10" For all bullets
.22 RF 14"* Twist for pistol barrels
16" Standard twist for rifle barrels
17"* Special twist for rifle barrels
.224 CF 9" For bullets heavier than 63 gr.
12" For bullets up to 63 gr.
14" For bullets up to 55 gr.
15"* For bullets up to 55 gr. driven 4,100 fps or more
16"* For bullets up to 55 gr. driven 4,300 fps or more
6mm/.243 8" Special for VLD bullets
10" For bullets up to 120 gr.
12" For bullets up to 85 gr.
13"* For bullets up to 75 gr.
14"* For bullets up to 70 gr.
15"* Special for bullets up to 70 gr.
.257 9" For bullets heavier than 100 gr.
10" For bullets up to 105 gr.
12" For bullets up to 90 gr.
13"* For bullets up to 80 gr.
14"* For bullets up to 70 gr.
6.5mm/.264 8" For bullets heavier than 120 gr.
9" For bullets up to 120 gr.
.270 10" For all bullets
7mm/.284 9" For bullets heavier than 140 gr.
11" For bullets up to 140 gr.
.307 13"* Special size and twist
.308 8" For bullets heavier than 220 gr.
10" For bullets up to 220 gr.
12" For bullets up to 170 gr.
14"* For bullets up to 168 gr.
15"* For bullets up to 150 gr.
7.65mm/.311 10" For all bullets
.338 10" For all bullets
9mm/.355 14" For low-velocity wadcutters
16" For all other bullets
.38/.357 14" For low-velocity wadcutters
18" For all other bullets
.358 14" For all bullets
.375 12" For all bullets
10mm/.400 16" For all bullets
.411 14" For all bullets
.416 14" For all bullets
.44 20" For all bullets
.451 16" For all bullets
.458 14" For all bullets
*Stainless Steel only
Black Powder Barrels (1.250" x 30")
.32 14" .320" groove
.38 20" .379" groove
.40 20" .403" groove
.457 20" .457" groove
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. An article in the 11/2001 Single Shot Exchange cites an article by Les Bowman in the 1962 Gun Digest offering an equation which includes muzzle velocity (in fps):
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.
Ken Howell wrote about twist rate in the 07/1999 issue of Varmint Hunter magazine. He mentioned Greenhill's work began with cannons in 1879. Two quotes Howell took from the Textbook of Small Arms (published in 1929 in Britain) are notable. "In actual practice Greenhill's figure of 150 can be increased safely to 200 and still control the bullet." The classic equation is for solid, lead alloy bullets of specific gravity (SG) 10.9, and "when the density of the bullet is less than that of lead or the density of the resisting medium is greater than that of air, the spin should be increased as the square root of the ratio of the densities." As SG decreases, the gyroscopic inertia of the bullet decreases in proportion, and one needs to increase the spin to compensate.
A comprehensive collection of interactive, online ballistics calculators! Exterior Ballistics, Recoil Calculator, Wound Channel Calculator, Stopping Power Calculators (mutiple), Round Ball Weight Calculator, Powder Calculators, and more! A one-stop resource for your technical ballistic data needs.
>> Permanent Wound Channel
Marshall's adaptation and refinement of a formula to quantitatively calculate the permanent wound channel potential of any load utilizing a bullet with a relatively flat frontal area. This calculator generates its results by utilizing the meplat diameter (in inches) of a projectile and the striking velocity of that bullet to give an outstandingly concise calculation of the true permanent wound channel left by the bullet in live targets.
>> Relative Penetration Calculator
Another Beartooth Exlusive found only here! Generates the relative penetration index for any given bullet. Calculates based on bullet weight and Meplat or fronal area of the bullet. A great tool for comparing the potential penetration of one bullet to another.
>> Thornily Relative Stopping Power
Peter Thorniley designed this formula years ago. While hunting extensively both in North America and Africa he did practical field testing on numerous kinds of game animals. Living on the family orchard, he perfected this formula by continued field testing while conducting extensive depredation work. It calculates via a relative stopping power quotient the amount of gun and load necessary to cleanly take a game animal under all conditions. A great tool for comparison of one load to another, as well as the potential of one cartridge to another.
>> Taylor Knock Out Power (KO)
John (Pondoro) Taylor Knock Out Power has been respected over the years as a great comparison of two cartridges, one to another.
>> Foot-Pounds Energy Calculator
Figure foot-pounds energy of a load.
>> Recoil Calculator
Ever wonder how much kick a gun has? How fast it comes back at you? This calculator figures the velocity in fps of how fast the gun comes back at you, as well as how many foot-pounds are traveling with it!
>> Ballistic Coefficient (BC) Calculator
Want to figure the BC's for a bullet? This calculator will do the long equation figuring for you! Maximize before use.
>> External Ballistics Calculator
Figure the ballistics on a bullet/load combination! Includes bullet drop, lead, and more. Maximize before use.
>> Loads per Pound of Powder
Calculate how many loads you're going to get out of that new pound of powder.
>> Sectional Density Calculator
Calculate the sectional density of your bullet. (Proportion of length to width)
>> Cost per Load/Box Calculator
Calculate how much it costs to load your own! Figure either per load or per box! Includes primers, powder, bullets, and more! Maximize for viewing.
>> Powder Burning Rate Chart
Lists 110 of today's available canister propellants. Helps to get an idea of how fast or slow a given powder really is. Also available in PDF format. Click Here for PDF. Maximize for viewing.
>> Round Ball Calculator
Calculate the weight of a round ball by entering it's diameter! You can choose from four common alloys- Wheel Weights, Linotype, Pure Lead, or 1-10!
THE FORMULA FOR COMPUTING WIND DRIFT
First some values:
t = time of flight in seconds
s = wind speed in mph
a = angle of wind off trajectory path
v = muzzle velocity in feet per second
r = range in yards
Wind Drift (in inches) = 12 x (88 x s / 60) x sin (a) x (t - (3 x r)/v)
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THE FORMULA FOR THE CORRECT SIGHTING OF A FIREARM
First some values:
H is the height of the sight above the bore in inches
D is the drop in inches of the bullet at sight-in range
S is the sight in range in inches
R is the range in question in inches
I is the drop from the bore at the range in question in inches
T is the difference in inches from the line of sight
If the range in question is less than the sight in range
T = (D-I) - ((S-R) x ((D+H)/S))
If the range in question is more than the sight in range
T = (D-I) + ((R-S) x ((D+H)/S))
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Kinetic Energy = ((Bullet wgt/225218) x Velocity)²/2
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Energy Transfer = Kinetic Energy x Caliber
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Optimum Game Weight = Velocity³ x Bullet Weight² x 1.5012 x 10e-13
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Mach = Velocity/1127
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Miles per hour = (Velocity x 3600)/5280
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Bullet Drop (inches) = 193 x (time of flight)²
(This formula does not take into consideration the effect of wind drag.)
Originally posted by Frogs
reply to post by DaddyBare
A quick question - two calibers for which there are good amount of both guns and ammo floating around here in the US are 30/30 and 30/06. I don't see them in the table - should they be assumed to fall under the .308 listing?
A good breakdown on bullet types would make a fine addition to this thread. Then we can discuss head spacing, crowning, floated barrels and glass bedding actions for those who want to go the extra mile or 10.
Originally posted by DaMod
reply to post by anumohi
I think DBare was just talking purely about ballistics.. Yes you're right that ballistics are not the only factors involved in accuracy but this thread is about ballistics man.