You are dealing with several factors here. There is no 'optimum' twist rate for any bullet weight. Each bullet has certain charactaristics of which weight is less of a characteristic than shape. A long tracer bullet requires a tighter twist rate for instance. Here goes...
Stability: To stabilize a bullet, very little twist is actually needed. That means, to keep the pointy end going forward. This has very little to do with any factor other than rotating the bullet. Virtually any rifled barrel will stabilize any .22 caliber bullet out of a .223 cartridge. Instability is often seen in pistol reloads and .22lr target loads as keyholing on target. You probably ain't gonna get that with a .223 round with the twist rates shown below.
Accuracy: The factors involved here are bullet weight, length, and shape as well as atmospheric factors such as temperature, altitude, and humidity. Generally speaking, the further you get from the most accurate twist rate, the more unstable a bullet will be and the less accurate it will be. This applies to either tighter or looser twist. Bullet weight is the biggest factor assuming we aren't using steel cored or tracer ammunition which are larger than their equally heavy lead counterparts.
Lethality: In general, you want the least amount of twist that will still give you acceptable accuracy. The slower the bullet rotates, the more likely it will want to fly 'fat end to the front' in tissue (of, say, a deer sized game animal). To fly that way, it will want to spin around and even fragment. You can't do this if you are spinning very fast. So, in general the slower you rotate the bullet, the more likely it will tumble in the [deer] and cause greater tissue damage. Of course, this is a tradeoff with accuracy as you still want to be able to hit the [deer] to realize the increase in effectiveness.
Penetration: This is inversely proportional to lethality when speaking of full metal jacket bullets. To penetrate deep and consistantly, you need a bullet that spins very fast and is stable even in tissue and other media. Tests showed that the modern 1-in-7" twist rate of the M16A2 causes the SS109 bullet to act like a 'drill' and create a nice, straight, tiny hole. If you ain't shooting [deer] through walls or sandbags, you don't need quite so much spin.
Okay, the numbers... there really aren't any hard and fast numbers. It all depends on your barrel length, velocity, barrel construction, what shape bullet you have, jacket thickness, etc. I can give you generalized numbers though of the best in terms of accuracy but understand that you have to increase the twist rate for penetration and decrease it for lethality. I'm pulling these out of my head as I can't find my books. I'll list the acceptable accuracy range for each load IMO. Bullets at either end of this range are likely to show poor accuracy. Remember, these are for homogenous lead bullets of boat tailed and hollow-point configurations. Steel tipped ammo needs more twist to stabilize.
1-in-7" Twist: 55gr and up. They don't, and shouldn't make a twist rate that's faster than this. Optimum weight is 69gr
1-in-8" Twist: 55gr and up. Mostly used on match grade barrels and optimum weight is about the same.
1-in-9" Twist: 52gr and up. You won't really notice a drastic loss in accuracy even at the top end of bullet weight but this is really a 62gr twist rate.
1-in-10" Twist: 50gr to 69gr. This really isn't useful unless you are shooting a mix of 55gr and 62gr bullets and don't plan on shooting anything else.
1-in-12" Twist: 40gr to 62gr. This is optimum for 55gr bullets but is really not that versatile with the heavier bullets. Use this if you are planning on using ligher loads or don't expect to ever shoot the really heavy bullets.
1-in-14" Twist: up to 55gr. This is strictly for varmits. Don't plan on taking on any [deer] hiding behind sandbags with this twist rate.
Remember, attack the opinion, not the poster. Hope this helps.
