I'm working on some pixel guns for a game that I'm trying to create.
for the first pistol have the gun barrel case extend past the horizontal bound you made; no pistol is designed with a handle attaching to the back since some part of the top has to kick back and load the next bullet in order to be semi-auto.
shotgun looks ok but you might want to think about the purpose of each part. exit end of the barrel is also really small.
revolver is just scaled entirely wrong, look at the positioning of the barrel to the cylinder:
the barrel is aligned to the top end; aka a position where the bullet chambers will line up. you can technically put the barrel to the side or bottom but that would just look odd, since you'll still need to include a mechanism for the hammer to hit the bullet.