Barrel length and twist rate uniformity

It is my understanding that gunsmiths generally use the Greenhill Formula to determine the optimal twist for a given bullet. The formula is T=150(d/r) for velocities from about 1500 to 2800 fps. Substitute 180 for the 150 value for velocities exceeding 2800 fps. "T" is the twist rate. "d" is the bullet diameter. "r" is the bullet length to diameter ratio (length of bullet divided by diameter). In .308, it works out fairly well. Sierra 168gr. Matchking is 0.308 inches in diameter. Bullet length is about 1.210inches so we have a length to diameter ratio of 3.929. Plugging this value into the formula and using the 150 constant (the proven best velocity for the 168 grain MK is 2550 - 2600 fps), we get T=150 x (.308/3.929); T=150 x 0.078; T=11.76 inches or 11 3/4 inch twist …. One rotation of the bullet for every 11.76 inches of barrel traveled. For the 175 Matchkings, again diameter is 0.308, Bullet length is 1.257 inches, resulting in an ld ratio of 4.081. … plugging into formula (optimal velocity is 2500 to 2550 fps so we will use the 150 constant again) T = 150 x (.308/4.081); T = 150 x 0.0755; T = 11.320. That approximate half inch twist difference makes a big difference in guns.

After you designate the twist rate for one full revolution, does barrel length factor into being able to further stabilize the bullet? Is there an ideal number of rotations the bullet needs to make before leaving the weapon? If I were to stick with a perticular cartridge, let's call it the 175 SMK, and I'm trying to maintain a uniform 2650fps, will using a 20" compared to a 26" reduce the velocity of the bullet? What about the 22"-24" barrels. Is there a particular size to effectively maintain the 2650fps.?

It is my understanding that gunsmiths generally use the Greenhill Formula to determine the optimal twist for a given bullet. The formula is T=150(d/r) for velocities from about 1500 to 2800 fps. Substitute 180 for the 150 value for velocities exceeding 2800 fps. "T" is the twist rate. "d" is the bullet diameter. "r" is the bullet length to diameter ratio (length of bullet divided by diameter). In .308, it works out fairly well. Sierra 168gr. Matchking is 0.308 inches in diameter. Bullet length is about 1.210inches so we have a length to diameter ratio of 3.929. Plugging this value into the formula and using the 150 constant (the proven best velocity for the 168 grain MK is 2550 - 2600 fps), we get T=150 x (.308/3.929); T=150 x 0.078; T=11.76 inches or 11 3/4 inch twist …. One rotation of the bullet for every 11.76 inches of barrel traveled. For the 175 Matchkings, again diameter is 0.308, Bullet length is 1.257 inches, resulting in an ld ratio of 4.081. … plugging into formula (optimal velocity is 2500 to 2550 fps so we will use the 150 constant again) T = 150 x (.308/4.081); T = 150 x 0.0755; T = 11.320. That approximate half inch twist difference makes a big difference in guns.

After you designate the twist rate for one full revolution, does barrel length factor into being able to further stabilize the bullet? Is there an ideal number of rotations the bullet needs to make before leaving the weapon? If I were to stick with a perticular cartridge, let's call it the 175 SMK, and I'm trying to maintain a uniform 2650fps, will using a 20" compared to a 26" reduce the velocity of the bullet? What about the 22"-24" barrels. Is there a particular size to effectively maintain the 2650fps.?

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