Another point to consider regarding barrel length and accuracy is sight radius. An 8" AR barrel has a *much* shorter distance between sights than a 20" AR barrel. Some competition rifles have an extra length of over bored tube attached to the end of their barrels just to extend that sight radius.
If you are talking scoped rifles, then a slightly shorter barrel of the same diameter is stiffer than a longer barrel and may actually be more accurate with all other variables equal.
Of course when you are getting down to 8" barrels in a rifle caliber, you are leaving a lot of powder to burn after the bullet has left the muzzle, which means lower velocities than that caliber is designed to be operated at and that can probably do some odd things.
I guess I'll have it cut down to 16" and have a great little carbine.
Yea, you'll be fine around 16" to 18". I'm not sure whether you are talking about a bolt action or AK/SKS, but I think the biggest problem with the AK and SKS is sight radius. If you could move that rear sight back 3"-4", it makes a big difference. Case in point: The
Tech-Sights rear sight upgrade for the SKS. It made a huge difference for me and others have said the same thing here.
Personally, I always wondered what the difference in velocity was between a 16" AK and a 20" SKS. I have a gut feeling that 7.62x39 is slowing down in an RPK barrel after 20" but I can't find any hard data anywhere.