Indeed 16" is legal limit on rifle barrels.....As to the question however, I'm not quite sure what you are driving at. I guess it depends what your intended uses are. As in the the .243 the velocity loss will limit its range when used for medium game when strictly looking at it's energy level in an debatable ethical point of view. I haven't looked at it, but I don't know that I would consider the .243 a 500 yd deer round, but it certainly has proven itself used on deer. Also set up right can be used at long to extreme range varmint/target round. I however use a .243 ai in pistol configuration with a 15" barrel and shoot the 55 grainers out to 500+ plus on a regular basis on varmints and such. And to a degree the same can be said for the .22's you mentioned. Looking at it from a point blank range perspective, not always but generally I find that with each 100 fps velocity loss, you see about 25 yds loss in pbr if the parameters aren't changed.
I agree also that barrel length has very little to do with accuracy, but most barrel manufactures I have talked and done business with say, it's a whole lot easier to keep a shorter barrel in tolerences.
Again what are your intended uses come into my mind.
Dave