Nomercy448

For a given cartridge, the bullet weight will dictate an approximate muzzle velocity it will produce. For that particular muzzle velocity, a given twist rate in the barrel will produce a specific rotational speed (RPM's) on the bullet. Each particular bullet weight (length, density, weight, and profile) will need a certain threshold rotational speed to stabilize.

For a 308win, a 180grn pill will run slower than a 150, AND it will be longer, so it'll need a faster barrel twist to stabilize.

In general, you have to try pretty hard to find factory ammo in 308win which isn't within proper specs for a factory twist rate barrel. 1:10" in a 308win will stabilize up to 200's most of the time, but nobody tends to shoot the 200's anyway.

Barrel length won't matter as much, in general, as the cartridge nor the twist rate, simply because there's not that much difference in performance (muzzle velocity with a given bullet weight) for a 20" barrel vs. a 24" barrel. In some cartridges, however, you'll see models with 16" barrels and other models with 26" barrels, where there might be 400fps or more difference between the two. In that case, a 16" barrel might need a 1:7" twist to stabilize a 77grn bullet, whereas a 1:9" twist in a 24" barrel might stabilize the 77's just fine (specifically use these as examples because I own rifles where this is true).

For lighter bullets, there's really no downside to running a twist which is too fast. They'll lose a tiny bit of potential velocity and run higher pressure than they could in a slower twist, but overall, you won't see any appreciable difference - as you're noting with your 150grn bullets.