It depends on the load really...if all the powder burns BEFORE exiting the barrel it will be fine, if not you are losing valuable "down range" performance.
You should have no problems at all with the .243 20" barrel combination.
I prefer a standard weight rifle for free-hand shooting myself, when I try to use a light rifle I actually fight the lightness to try to keep it steady, this could just be me...
Stay Safe. Happy Hunting/Shooting.