Like an
F2000 for the SARP. In fact, it has some of the same statistics (most notably weight).
The 7.62 x 51 mm is not a Soviet caliber (it's also NATO), but it doesn't matter -- the SARP already has a rifle that uses that caliber, so it gets to stay (but as 7.62 x 51 mm KZ).
5.56 x 45 mm goes, however.
The .50-caliber Handgun caliber -- that goes to the HHG, right? If you really want it, you can keep it. It will have less recoil than the 7.62 x 51 mm round, I guarantee that, but I have to ask, why even have that as an option? The ammunition will be scarce and there's nothing it can do that 7.62 mm (.30-caliber) can't do better.
The shotgun idea Jessica proposes is a good one. You'd have plenty of room to include BOTH of those weapons with a little change in the art to show the second barrel. It's a very bulky weapon right now, weight withstanding.
If you can, do away completely with the changeable receivers. No soldier's going to want to deal with that.
The ranges on the 7.62 round are ... well, you're using a 27-inch barrel, which is pretty good for a small assault rifle. The problem is that's impossible -- you're not accounting for the rifle's internal bolt, which should shave off at least six inches off your barrel, if not more. That cuts your barrel down farther, but theoretically it could still hit a target 550 meters away. Just don't expect it too often -- the old
GP1 is still a better sniper weapon.
You realize this thing's not ambidextrous, right? Steal from the F2000 and put the ejection port forward of the gun, below the barrel. Easy thing to draw in.