Ok, it's time to finish this one, after a looooot of hours spent on it 
I learnt a lot of things... It's my first one and I will probably make another weapon in the future... maybe some things to make it better but NOW, I'm done ! 
It's about 8000 tris. The pump and the shell system can be rigged for animation but that's all. Texture is about a diffuse, normal and specular map (with gloss in alpha) 2048 x 2048. Can be used for a fps I guess 
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