I don't think this has something to do with the mission it's self. Same thing happend to me wenn i tryed to take a 6.5mm Silencer out of a custom Ammo Box in a mp mission i had setup my self. (no mods only bi attachments in the box, no mods in the entire game also)

Win 7 x64
Alpha 0.5.102571 normal build (not the developer build)