Re: D20 Modern: Rules for a M45 Quadmount?
Never used the system myself, so I figured, what the heck, I'll take a look.
Just looking at core SRD from Wizards, it all seems to be balanced by fire rate/Ammo capacity outside the damage. The machine guns using linked ammo, assault rifles using mags with limited ammo.
Therefore, you are probably right, it would not be different unless you want to brew up something like dual wielding rules perhaps?