I know and prefer round ball works better in traditional longrifles.
However is there a formulae to calculate projectile length & weight, velocity, and rotation speed to achieve stability in flight of a "bullet".
ie: why is 1:60 good for a .45 rb, and a .45, 405 grain bullet need a 1:22 twist
I know there has been many hours of study to achieve this information, but is there a formulae?
This question relates to the use of R.E.A.L. bullets in the .40 cal. in a previous discussion.
Projectile weight is not a factor in finding the needed twist, only length.
The Greenhill formula was developed for rifled artillery and has proven to be only ball park for small arms. It will produce a twist that will work but its off about 2" for BPCR with heavy bullets, 500-550 45 for example. The needs of artillery are different than for small arms.
For example departure angles and the angle in relation to flight path when the projectile arrives on the target. This can effect penetration on hard targets. This is why the M1 Abrams tank has a smoothbore gun that shoots "darts".
The original Minie is nearly a dart, like a Foster slug used in shotguns, and will give military accuracy in a 72 twist as a result. The Brennke slug has wads permanently attached that act as fins. The minie does not compute with Greenhill.
A high departure angle an cause some projectiles to become unstable past the mid-point of their trajectory at long range when the "relative wind" begins to impact the projectile behind the nose rather than on the point. This is apparently more pronounced with highly streamlined bullets like the 155 gr Palma Bullet used in the 308 at 1000 yards.
Very short bullets like the REAL shoot fairly well in slow twists, I found years ago that even a 54 Maxi-ball would stay point on to 150 yards shot from a 72 twist. The REAL has been around for a long time and when it was originally marketed "modern" MLs with twists faster than 48 were unknown.
So far as the 40 Real bullet in practical use. It would be similar to a 38-40 or the more powerful 40-65 WCF for hunting. But like the maxi, the "ball-et" and the others these were developed to fill a PERCEIVED need by modern shooters and gun writers who were just industry shills, who believed the round ball was incapable of either accuracy or killing game. Or they wanted to hunt big game with a rifle that was too small in the bore.
But the 40 REAL will not outperform a 200-220 gr RB on game. For that matter at the long bow distances many people shoot in the east from tree stands and blinds the 40 RB will work. I am sure that back in the day a lot of game was killed with head shots. But with the arrival of Chronic Wasting Disease in some parts of the US brain and spinal column shots are no longer a good choice.
People who insist on shooting elongated projectiles should get a barrel with a twist that will shoot bullets PROPERLY.
Find the bullet they wan to shoot then buy a barrel to match. If I were to do this I would find a twist that would work for a bullet like the Lyman 456192 then have the barrel cut with a .455-.456" BORE so the bullets could be run through a luber-sizer after being cast. But finding someone will cut one from real gun barrel steel will likely be a problem.
Short bullets will stabilize at hunting ranges in twists from 25 to 60 inches depending on caliber. Some of the Win. 50 express rifles had twists in the 56-60" range but were not noted for their accuracy. Most of the short bullet cartridges like the 40-65 had twists in the 25" range. Its in Cartridges of the World if someone is interested.
Here is a site with a calculator.
http://kwk.us/twist.htmlI put in numbers for a 45 I would use a 18 twist in, 1.5" long, 1300 fps and it gave me a 17.6" twist. Sharps final LR rifles used 18 twists in 40 and 45.
It gives 83" for a .535 RB at 1900 fps.
So measure the length of your bullets and go for it.
Dan
