This is how I usually design weapons, just black silhouettes at the start.
This was my first pass (done a few months ago) for weapons on Potemkin. I really hadn't done any concepts in a long time so I was still pretty rusty.
Most of them aren't being developed much further or are have been dropped. For example the Gravity Hammer was fun to design but just doesn't fit the game. Also wanted to go in more of a direction of having non-weapons as weapons instead of handing the player 10 different assault rifles in a horror game.