it being a robot..anatomy doesn't really come into it! as for the structure of the character, it looks fine. might want to change the angle of the rings around his back leg (our right) so that it looks a lil more on perspective and the hand holding the gun is massive but again, he's a robot, he can have massive hands! also, does he have an antenna on both "ears" if so we should see the tip of the other one!
as for the overall piece? it's great! not to much of a fan of the muddy texture behind him but this is solid work my friend! reminds me of a piece pennyarcade.com did for a noir/futuristic comic, as in the colour scheme! i remember thinking i would love to read awhole comic in this style! looking forward to more from this world!