I guess you used LoadAnimMesh for the Gun? Well, probably you simply want or need to use an animated Mesh for the gun, as I do too. I had this Problem too, when I worked on my first FPS Demo. My solution was to downscale the weapon that much that it never would sink into a wall or something. Of course, this also requires to position it closer to the camera center. Probably, when the weapon looks too long-stretched this way, you need to scale the depth of it even smaller than the rest (a camera-zoom issue)