Greenhill Formulae

[FrankG]

Thanks razor for posting this as it will help determining rifling twist for what barrel will stabilize ball or bullet.

One of the first persons to try to develop a formula for calculating the correct rate of twist for firearms, was George Greenhill, a mathematics lecturer at Emanuel College in Cambridge, England. His formula is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers. This can be simplified to:

    Twist = 150 X D2/L

    Where:
    D = bullet diameter in inches
    L= bullet length in inches
    150 = a constant

Heres a calculator I found.

http://www.mountainmolds.com/helpGreenhill.htm

[Ironwood]

Hey Frank,  Thanks for the website.  When I plugged in 1-48 twist and .400 bullet diameter the calculator came up with .600.   It looks like the 1-48 twist in my Green Mountain .40 caliber barrel will stabilize the Lee 40 caliber cast bullets.  Gonna order a mould and see how it works.   

[FrankG]

That was my thoughts also Only thing is if to large they will need a size die and if too small a bump die . But I have lathe and have made them before  ;)

[Ironwood]

Frank, which bullet do you think might retain the lube better?  The 145 grain has a big lube groove, but the 175 grain has lots of small grooves.  The 175 grain might be easier to load.  What do you think.  Anyone else got any ideas?

[FrankG]

I like the looks of this'n .    www.leeprecision.com   90256

[Ironwood]

I'm worried about the lube groove on that one.  My 40 has a 32 inch barrel.  I'm really a little worried about the lube on all three of them. :)   

[FrankG]

My Sharps has a 34 " barrel and I only lube the bottem two narrow bands as I was getting a wax ring on muzzle from excess lube. I use SPG lube for BP cast bullets . I think its a lanolin / beeswax lube. I think this lube would work well on the little 40 cal . I believe the more vicous lubes would be spent before reaching muzzle .