Hi Im Just wondering if anyone could explain twist rates in a rifle barrel for me? For a benchmark could we go with a .308 , now if I wanted to shoot 90gr pills flat out whats the ideal twist? and if i wanted to shoot the same pill subsonic what would the ideal twist be then?,also what Is different If i wanted to do the same with a 220grain pill? I think my rifle has a 1 in 10 twist(not to sure) and am toying with the idea of some reduced loads for some can shooting and letting the wife have a go but was told my rifle wont shoot straight if I slow 220grainers down to much Is that true?,I was given over 1000 projectile that are 220grainers thats why i want to go with them thanks.

To cover all possibilities and minuta would take a book load of advanced mathmatics.

Or you could look up the Greenhill formulia and go from there. Yes, velocity does make a difference, as it's RPM of the bullet that is important (RPM being a product of twist rate and muzzle velocity).
Note that increasing the "constant" by 20 for magnum velocity cartridges and reducing it by 20 for "slow" cartrdges will yield better results, as the "constant" is calculated for "normal" hunting velocity type rounds.

Yes thanks I found the green hill formulia and have abit of an understanding of the contributing factors to instability now thanks,i will just have to do the usual work out a load for these pills but start at moderate speeds to keep the recoil down and noise down thanks again

As you have learned, the length of the bullet dictates the amount of twist required. The 220 gr. bullets are round nose, and so are shorter than spitzers (pointed bullets); the weight is not the determining factor. BR-shooters won't fully agree, but as a rule of shorthand, you can't over-stabilize a bullet. (The exception being a thin-jacketed, lightweight bullet that is driven at high velocities; the tight twist may shear the jacket off.)

Your 220 gr. bullets need a 1-10 twist to stabilize. 308's usually have a 1-12 twist rate, meaning that you should NOT expect the 220's to stabilize. On the other hand, load up a handful and see if they keyhole the target.

How fast they shoot does not affect stabilization very much (here, the BR-shooters may pose an argument). For your purposes, it is not a factor.

If you're looking for low-recoil loads for your wife, invest in a box of 130 (or 90) gr., or something like that. The bullet weight has as much effect on recoil as the velocity (impulse and momentum).
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