I would design a mechanism that allows the operator to choose between open- and closed-bolt operation, that way you can decide between a open-bolt for lower chance of overheating, or closed bolt for harsh enviroments where clogging is a problem. Short-stroke gas piston system similar to the M6A2 PSD
Either 6.8mm or 7.62mm NATO Caliber, If a 6.8 it would feed from a 30 round box or 100 round Beta C-mag, if in 7.62mm NATO, it would feed from a 20 round box or 80 round drum.
The gun would probably be a IAR(Ifantry Automatic Rifle) With one of those foregrips that doubles as a bipod. M-16 pistol grip with Magpul stock and Quad-rail handguard. 1.5x Red Dot sight and a two-position gas valve regulator, choice between 600 rpm or 900 rpm.