[m21sniper]

The USMC wants to replace some M-16A4's with some new fangled squad automatic rifle that will automatically switch to open bolt operation when it heats up, so that they can increase the volume of fire of their units.

There are things that can be done to improve the M-4s ROF significantly, but they would cost quite a lot of money. Conversion to open bolt firing and the adoption of a thermally coated fluted heavy exotic material barrel would be the most impactful, but it would probably add about 30-40% to the cost of every rifle.

Another thing that would improve sustained ROF is simply switching back to the full length M-16A4. That would also increase range and hitting power, but the Army performed in the field tests of the M4 and the troops overwhelmingly favored the carbine over the full length M-16, that's why the M-4 was adopted to begin with.

Any weapon is a design compromise, the M-4 is no different. It is good at some things, and is not at all optomized for others.