What material are you using to create the weapon? I think for this small scale you will struggle with any type of putty, best bet is to create the weapon from styrene sheet/tube etc and maybe some thin gauge wire for the barrel.
Its best to scale an image of the actual weapon as a template to work from, this way you will maintain scale and get each component in its relevant area.
try to break the weapon design into basic shapes and use those shapes as a starting point to create each section.
Steve