[Alleycat]

Originally Posted by LouBoyd

I own both LR-308s and 300 Whispers. None of them has or needs adjustable gas blocks or needs special buffer springs. I don't attempt to use supersonic loads in the 300 Whisper or subsonic loads in the LR-308.

There are a lot of reasons not to have a "does it all" subsonic/supersonic ARs. The main ones are:

different optimum barrel twist rates
different barrel length for desired performance
large difference in gas system Over 5 times difference in
available propellant gas.
Difficultly in achieving low velocity dispersion using excess capacity cases.
Wimpy performance from supersonic 300 Whisper compared to a 308 with no benefit.
Different throat requirements for light supersonic versus heavy subsonic bullets.
Different scope choice and different sight adjustments for super and sub sonic shooting.
Different suppressor requirements (if used)

By having separate dedicated rifles both can remain set up and and immediately ready for use.

http://en.wikipedia.org/wiki/George_Mallory