Greenhill formula: Overall length or "Bearing" length for projectile?

[Tim Lyne]

Esteemed Riflemen: Mr. West is churning chips on a 2.25" Confederate Mtn. Rifle for Knap's Battery. While the tube is being birthed I'm working on the projectile, which brings me to Mr. Greenhill's formula. Since he designed this for Artillery in the first place it seemed like the right thing to do. With a bore of 2.25" and a twist of one in 10 feet (120 inches), I'm getting a projectile length of near five inches based on fps in the range of 1000 to 1100. Would I be right in assuming the suggested length is bore-bearing surface length and not an "overall" length? I'm working on a pellet design so it does make it an important consideration. I'd appreciate your advice! Tim Lyne, Adj., Knap's Battery Comp. #2952V

[John Bly]

Tim, the hill formula applies to the overall length of the projectile. It is a length to diameter ratio formula to determine a length to keep the projectile stable in flight and it really has nothing to do with the bearing surface in the bore. You can use lower numbers than the 150 in the formula. Some use 125 today for modern bullets for use with black powder. I hope this helps.

[Fearless Frank]

Pellet style can be good but if you make it too long and keep the weight requirement the head will be too thin and you might get a noodle.

[Tim Lyne]

Yes it does appear to be a bit of a dance. I recalculated the length based on a FPS of plus/minus 850. That puts it a little shorter and should help keep it nose heavy. We'll see! Tim, Knap's Batt.