ok I have made a few observations so bare with me, infact heres a marked up copy of the image and I will just give you a quick key to what it means
RED: These areas need a sharper fall off instead of a curved edge
BLUE: The cog looks like it should be a octogon (8 sided) shape in order for easy twisting when out in the action
YELLOW: The holes in the muzzel should be see through with no pipe in the middle, this allows for some ventilation to reach the muzzle
GREEN: From the concept it looks like there is some kind of switch or cocking handle which I can't see on there
Also it would benifit from a less of a rust looking texture make the patches bigger and more sprase in order to give it a camoflage feel
but all in all a good job