20" barrel will give you more velocity. Heavier barrel with longer sight radius will give you more practical accuracy.
I got the thing because I want to use it as a rifle, i.e. to shoot targets at a fair distance. A carbine works fine, too, for its intended uses -- e.g. the Army uses carbines, the Marines use rifles.. The bullet just won't be going as fast, but that probably doesn't matter if you're thinking about the ranges involved in defensive use. At over 200 yards, it's probably called "murder", not self-defense.
The bullet slows down as it flies, either way. So, say a particular bullet mushrooms, fragments, or whatever, at 2000 fps or higher. That translates to some distance from the muzzle; the faster it's going at the muzzle, the farther away it will still be going 2000 fps.
Why do you want a CMMG, in particular?