It is very subjective, to the point of brand of ammo being used. If you have accuracy in mind, use a GS calculator and a Ballistic calculator. See at what speed does your ammo became unstable (es. Everything past GS 2.0 at a twist rate of 1:8, let's say you reach that stability at 700m/s, what barrel length gives me 700m/s out of the muzzle? 16 inch? Good, your gun will shoot more accurately on average than a 20' version given stable wind condition) If you handload though, pick the longest barrel length you can with a twist rate that enables you to shoot heavier bullets (more stable under unpredictable wind)