Twist

[NorCalRifleman]

I'm shopping around for a .22 centerfire, probably a .223 or .22-250 and was wondering how important rifling twist rate is. What is a 1 in 9" twist better for as opposed to a 1 in 12"? I'm going to start handloading and will use bullets of varying weight for game ranging from ground squirrel to coyote. Anything bigger will get a dose of .30-06.
So how does twist rate effect accuracy and terminal performance? How important is the right twist rate and what rate would be best for a multi-purpose rifle?

 

[Vern Humphrey]

Twist rate determines how LONG a bullet you can shoot. The old Greenhill forumla is (Length^2 X 150)/bore diameter.

Very fast twists are used in small bore centerfiress to allow them to shoot long, heavy bullets, either for target work, or for shooting large critters (terrorists and deer.) If you intend to use yours for varmit shooting only, a moderate twist and light bullets will serve you very well.

One disadvantage of very fast twists in small bore centerfires is that if you drive the bullets too fast, the forces involved can rupture the jacket and the bullet will literally disintegrate in the air. This might well be a problem in the .22-250.

 

[1911 guy]

What range?

I'd decide that before deciding between .223 and .22-250. Inside about 300 yards you're good with a .223 for 'yotes. More for smaller stuff.

Going by the Greenhill formula (much simplified so a rube like me can understand it), a .22 bore with a 1:9 twist should be optimum with a bullet length of...lemme get a cipherin' machine and edit an answer in.

.8068 inch projectile. Hard to stick down a grain weight due to manufacturer variations in jacket thickness, material, etc. I've got some hornandy 68gr HPBT's about that length, though.

 

[Navy_Guns]

Formula error

Vern switched around the length and diameter...

twist rate = (diameter^2 x 150 or 180)/length

 

[Jim Watson]

You gotta be careful with the Greenhill formula. It was empirically derived for rather blunt bigbore projectiles with velocities in the upper "teens".
Note that Navy gives you two different constants, the original 150 and 180 which is supposed to do better at higher velocities. I have also seen BPCR data with a 125 constant so as to give adequate stability below 1500 fps.
Looks like circular reasoning to me, and reason enough to go by more modern sources, like barrelmakers and more experienced shooters.

Were it me, I'd get a 12 twist and maybe that 60 gr Nosler for coyotes. About anything for lesser varmints.

(And I own a 6.5 twist .223 for shooting 90 grain bullets at Long Range, so I have been at the fringes.)