One thing that comes to mind is the design of the bullpup-type AR's.
They need to be slightly longer at the rear, behind the magazine, to allow the mechanism to fully repeat. Right now you could manually load a round directly into the barrel and upon firing the mechanism would hit the back of the gun, without loading the next round or ejecting the casing. However, if they are using caseless ammo they should work just fine (hint-hint) :)
Aside from that, the design looks nice and rugged.
You could most likely use the Enfield L85A1 for reference.
Great work otherwise, really like how the story is developing.