As you can see on the picture, there's a flying object near the main weapon. It's the part of the weapon that should look exactly like on the smaller picture - it's the GECK render but it appears correctly also in the game - in 1st person view is mesh bugged too.

Please help.

http://www.imagebam.com/image/c11b35146667248

Check the blocknames in NifSkope, it could have the same name as some of the moving parts.