Youre thinking about it from the standpoint of caliber, not functionality.
What is the function of a squad support weapon? To make the enemy keep his head down to enable the rest of the squad to maneuver around his cover.
Automatic fire isnt really necessary. 300 rpm would be perfectly suitable, and that is only 5 rounds per second. You can probably approximate this from a semiauto if you dont bother aiming each shot too carefully. The challenge is in making the gun transit quickly enough to peekers rather than sending enough lead their way.
The real question is how much caliber of bullet is necessary to make enemies keep their heads down, and how many of those bullets can you fire from a single magazine without reloading or melting the gun.
If 223 is enough, you can probably use an AR15 variant with a beta mag. If 7.62x39 is more your style, there are a ton of high capacity drums available for cheap. If you want a full power rifle cartridge you might want to break out the checkbook for one of those semi-auto belt fed M1919s. They can be fed 308 and will go through a ton of rounds very quickly. With an israeli style shoulder stock, they would make an OK squad support weapon. You can go prone with any of these guns easily and they all have capacity of 100 or more rounds before a reload (they will all start to cook off at this point anyway, especially the assault rifle variants).
Actually, a semiauto M1919 with an ACOG and a bipod would make a wicked support gun though not useful for much else IMO.
