Hey dude, nice concepts. I avoid concepts, my vision is better than my sketching abilities, but that's no excuse
For me, the (edit- very first drawing), with a rifle, would just be pure awesome. I love his shape, dynamic and powerful. I can just see him in a pose with 3 hands steadying that rifle, standing in the dead center of a war zone, because he's too tough to need cover
I also like the larvae artillery, very creative, but I don't know it will have the same visual impact.
Good luck