why is my Gun Model Only rendering the front bit when the animations are on?

Could someone please please PLEASE, help me out here, Ive created my gun and textured it in max, ive put it into unity and i can now run about with the working gun etc. However when i set a loop animation too play for the gun at idle, it is only showing the very front bit of the gun and the rest is invisible. Im running the gun out of two files. The actual machinegun file, then my secondary folder of files for the animation. If i set the Machine gun file to run the animation from the secondary file then i have this, only displaying half the gun, problem, however, if i run the animation off the same file, the gun will be entirly invisible. This is confusing as they are even the same model, both hold the same animation and the second is simply a double of the first, so why would it 1. Not display the actual gun when animated, and 2. why would using the double have a different affect. (despite being equaly usless).

Thank you, I would really appreciate any knowledge shone on the situation.

When making an animation you have to bind all the vertices to the bones. If some vertices aren't bound to any bones they will disappear into infinity. Possibly this is the problem you're experiencing.

If you only see half the gun, I bet your near clip plane value is too large. If you don't see the gun at all, I think it's because you don't have "Update When Offscreen" checked. The reason it would be different is a matter of pivot point.