In a recent conversation with a very knowledgeable single shot rifle shooter, I mentioned that I had calculated the maximum bullet length for the breech loading rifle we were discussing using the rifling twist, etc. and the well know Greenhill Formula. 

I was promptly informed that the Greenhill Formula applies only to muzzle loader rifles and, therefore, I should disregard the bullet length that I arrived at using said formula when selecting bullets for my single shot rifles.

Was my mentor correct with his guidance for me, and if so, why would a muzzle loader shooting a conical bullet be any different from a breech loaded rifle?

Thanks 38 Caliber for that input.  The constant that I use is 150, are you saying that I maybe should be using some other constant?

Creed Moor

The constant factor for best stability depends on how far you are shooting.  For example for 1000 yard shooting, I like a constant of 125 which is a better bet than the 150 in the standard formula.  Another factor that affects the constant is the BC of the bullet in question.
Keep on hav'n fun!
MikeT

Greenhill developed his formula when lead bullets from a trapdoor springfield was state of the art. So the constant of 150 better fits lead bullets at moderate velocities than modern boattail spitzer bullets at >3000 fps. The formula was developed empirically - just trial & error to fit the observations, so others have modified Greenhill to fit more modern calibers.

One modification of Greenhill replaces the 150 constant with 3.5 times the square root of the velocity. At 1400 fps this gives a "constant" of about 130, and at 3200 fps it gives 200. This gives reasonable results for everything from a 32-40 to a 6 ppc. 

The modified formulas is online at:   (You need to Login or Register to view media files and links)

Regards, Ron

I dont have a real mathematical grasp of the Greenhill Formula.  My interest is success in long range competition using Black Powder Cartridge Rifles.  Which, in turn, necessitates a search for what works at long range in the rifles used. 

I have shot a few different rifles in .45 Caliber, different twist rates, Black Powder, different velocities, using the same bullet cast from the same alloy.  Specifically, the Saeco #745 Bullet because it has proven accurate over the course from 200 to 1,000 yards in all three rifles in comparison to other bullets tried...  Using an Original Trapdoor, a 1x18 Twist Badger Barrel and a 1x16 twist Douglas Premium Barrel at fairly similar velocities, (1285-1320).  In reasonably calm conditions it is difficult to tell much difference out to 800 yards.

However, when shooting in turbulent conditions, and/or beyond 800 yards, the 1x16 twist barrel does seem to resist turbulent conditions and keep the bullet more stable than the other rifles at the longer ranges, with this particular bullet.  By stable I mean that the bullet strikes on paper are more consistent with less random tipping and in constant conditions the bullets also strike closer together on paper.  It also seems that sight corrections are more predictable as to results on paper.

Observing this difference causes me to suspect that the higher rotational spin imparted by the 16 twist might aid in keeping the projectiles rotational integrity intact throughout flight to the target at the longer ranges.  I feel that this is important because each shot fired is at least in a slightly different condition and sometimes in a very different condition and this, more than anything else, seems to determine where the bullet strikes on paper at 1,000 yards and not any other factor.

I am tinkering with the same rifles using a 560 grain bullet and it will be interesting the see the results.