twist rate vs. bullet weight question

[Scibaer]

what is the basic correlation between the twist rate and the weight of the bullet ?
 i have a 1:20 twist barrel ( .44 ) i have not slugged the barrel yet but would like to know the optimum bullet weight ..
is there a list or something ?

[fr3db3ar]

Twist relates more to length of bullet.  Sometimes you can shoot a heavier bullet because it is heavy for it's length.

[Lone Star]

There is a "something".  The Greenhill Formula can be used to determine the longest bullet usable in a given twist.  It was designed for all-lead bullets - not jacketed ones - but it will come close. 

maximum bullet length = (diameter x diameter x 150) / twist

.

[Scibaer]

so do i read that too mean, that heavy bullets generally work better with slow twists and light bullets work with fast twists ?

got it, the greenhill formula, i will take a good look at that.

[searlock]

so how does this formula work with my .500 handi 1 in 18.75 twist?

[Scibaer]

i dont exactly know. i did find that this formula was designed for cannons in the 1800's and there are several different versions of it.
but the basics are, the length to weight ratio of the bullet, multiplied by 150 gives you the relative twist.

heres the thing, this formula was designed for round cannon balls with a very high sectional density, not rifle bullets with a long ogive and relatively low sectional density. done right it should come real close whether using rifle or handgun bullets, but i have not found which forumla ( there are several variations ) is most correct.

secondly, you'd want to know what length or weight bullet would shoot best in your barrel of whatever twist. you not going to be swapping out barrels or changing the twist rate, so its the bullet you'd likely change. but in relatively close distances ( 100 yard zero ) i dont know how much difference it makes.

it your shooting 500 or more yards it may matter, but at handgun yardages, it most likely dont. after looking at the different formulas and looking at ballistic charts, im inclined to think that bullet weight to twist rate matters more.  meaning slow twist for heavy bullets and fast twist for light bullets, or whatever is correct.. that could be the other way around fast twist for heavy bullets.

[knight0334]

so do i read that too mean, that heavy bullets generally work better with slow twists and light bullets work with fast twists ?

got it, the greenhill formula, i will take a good look at that.

Faster bullets require less twist
Shorter bullets require less twist
Slower bullets require more twist
Longer bullets require more twist


With longer bullets, generally they are heavier, which compounds the need for faster twist rates because they are also slower from the same cartridge.    An all copper .224" bullet may only way 60gr, but it is as along as a 90gr or more lead slug.   A 1:9" to 1:12" will generally stabilize a 60gr lead bullet at 3000-3300fps,  but 1:9" would have a hard time stabilizing an all copper 60gr bullet that has the same length as a 90gr lead slug.

Velocity is why .22-250's and 220 Swifts have 1:14" and 1:12" twist rates, when .223Rem have 1:6" to 1:12" twist rates.    Faster bullet of the same weight requires less twist.

[Scibaer]

thanks Knight, that really helps.
 i was looking up twist rates and the greenhill forumla and came across this
http://www.jbmballistics.com/~jbm/cgi-bin/jbmstab-5.0.cgi

its a bullet stability calculator, by JBM based on the Miller Stability Formula, reported to be more accurate and modern then the greenhill formula.
something to play around with, if nothing else.
glenn

[calvon]

The Greenhill formula makes no provision for velocity and for that reason alone it is probably suspect.

In Rifle Magazine, Nov-Dec 1994, Culp provides a twist formula that does include velocity.

(.06 × Muzzle Velocity) × (Bullet Diameter in inches squared) ÷ Bullet Length in inches.

[securitysix]

There is a "something".  The Greenhill Formula can be used to determine the longest bullet usable in a given twist.  It was designed for all-lead bullets - not jacketed ones - but it will come close. 

maximum bullet length = (diameter x diameter x 150) / twist

.

OK, if I'm doing this right, a .30 caliber (.308" diameter) bullet fired from a 1-10" twist can fire a bullet up to 1.42296" long, assuming one can also get the velocity high enough.  Dropping the same caliber to a 1-14" twist reduces it to a maximum of 1.0164".

Interestingly, the 155 grain Hornady A-Max bullets I have sitting here measure 1.218" from base to tip, but the box recommends a twist rate of 1-14", which makes me suspect the length from that formula is probably either best measured from base to ogive, or best used as a tentative guide...or I'm doing it wrong. 

[Badnews Bob]

You can play around with those formulas all week and they should get you close but you will never know if it will work or not untill you put one down the barrel, And just because it will shoot well out of my rifle don't mean it will shoot good out of yours.  I buy bullets I wanna try and shoot them if they work I use them if they don't I try something else. I never do formulas. I don't like math that much.

[Scibaer]

thats a real good point Bob, and your right. what it boils down to is knowing what bullets are worth buying and trying.
 i have bought some bullets that just didnt work, and i though it was me, i could not work up a good load.
the greenhill or miller formulas i was interested in using to help pick out better bullets, but the bullet weight to twist rate works well enough to pick better bullets .  now i will be wasting less money and hopefully picking out better bullets and building better loads ( i need a chronograph though ) .