A tip from a real-life grunt: the two weapons you have should fire the same calibre/ammo type. If you run out of heavy weapon ammo, you're gonna want to be able to scavenge from your SMG ammo, and vise versa. This is generally a good rule to follow from the individual grunt to a platoon-sized element--soldiers die, weapons break, you have to be able to interchange ammunition at the drop of a hat. Same goes for batteries, armor, screws, bolts and tires.
Now, if I were that Merc, I'd go with 10mm for the Pistols as well for commonality of ammo. Make supply a bit easier, and allow cross-loading if something happened to the SMG/pistol in the field. But that's me.
Quite a heavy load he has to carry... the standard magazines + the machinegun + the heavy armor. The look of the SMG reminds me of an UMP45, the Clipsize is the same as well. I think it's unusual that someone carries his knife at the inside of the leg. I really like your Idea of the Smart Frags. Oh, and of course i like the look of the whole artwork and the many details you put in it. Would be glad if i could paint like this.