barrel twist rate

[.270]

so the '06 bullet question don't get fogged up anymore than it already is. this is the Greenhill Formula, used to determine barrel twist rate. snagged it offa sniper website. this also a reason a guy need to stick with one bullet weight in a given gun. unless you shoot a .270, 1 in 10" is fine for everything. in .30 cal, .308, there is almost a 2:1 difference in barrel twist from the lightest to the heaviest bullets. you can't hardly shoot big and little bullets out of the same barrel and get any kind of accuracy. that's why a guy should stay around the middle in bullet weights on most .30 cal, because the barrel is probly twisted to stabilize the middle weight bullets. if you're gonna shoot great big or little bitty bullets out of most rifles, you're gonna hafta get a custom barrel and just shoot one weight. this formula works well for all calibers, but the .308 diameter is what is entered here. Lark.

 

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 168 grain Matchking is 0.308 inches in diameter. Bullet length is about 1.210 inches 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.