My main two 243s have 20" barrels--the only drawback is the noise level. They shoot great and don't lose that much over a 22".
On the other hand, I would not cut a mountain rifle down to 20", as you will lose the balance. They are perfect the way they are. If you need to lose more weight on the MR, go aluminum bolt shroud and Rifle Basix aluminum trigger. Then put a flipflop recoil pad on.
