Bob,
Ok.... Here it is. I make no guarantees that this will work any better than any of the other formulas out there.
These formulas are about like lubes. One for every shooter! A Maj. Cundill & Mr. Hancock also have their interpretations of Greenhills formula listed to.
What I've printed below is a direct copy from the letters, so spelling and sentence structure might make it a little hard to understand, but hopefully the math examples will make it clear. This comes from Lowe's daybooks and as such were more something to jog his memory than to be absolutely clear to someone else reading it.
PETE
Take the calibre of projectile (or diameter) of the bore of your rifle in hundredths of inches and multiply by 3, then multiply the result by itself; then take the length of one turn in rifling in inches in gun rifle and multiply that by 7 divide the first sum by the result of this and it will give the length of projectile
in hundredths of inches that can be shot from that rifle.
If you have a certain length of projectile and you wish to know the length of turn in rifling to shoot this projectile and keep it right end to, take the length of projectile in hundredths of inches and multiply by 7 then take the calibre (or diameter) of barrel of rifle it is to be shot from multiply by 3 and the result by itself and divide by result of the projectile; this will give the turn in number of inches (cut off the fractions and leave it even inches).
In small calibres with shallow grooves a little longer projectile can be used, for the weight has not been cut away on the surface.
Hollow bases must be cut down flat before measuring.
Sample calibre .32 .32 x 3 = .96 x .96 = 9216
Rifling 16 inch – .16 x 7 = .112
9216 divided by .112 = .8228 or discarding the extended decimals .82 the length of the projectile.
(32 x 32)2 =
9216 = 82+ or .82 length
16 x 7 112
Rule I
C = calibre taken in hundredths
T = twist in inches for one turn
L = length of projectile in hundredths of inches
Then
(c x 3)2 L measured in hundredths
T x 7
Rule two T is taken in hundredths of inches
Then T =
(C x 3)2 = 32 x 3 squared =
9216 = 16
L x 7 82 x 7 = 574