I could absolutely fabricate rifle with no sights. Also, the M6D pistol in Halo is a magazine-fed semi-automatic with a slide... not unlike a modern Glock, to be honest. The Needler falls in the range of "impossible but accepted because it's a sci-fi universe", the same as Ice Breaker, the Vex Mythoclast, etc, etc, etc... Every human weapon in Halo could be fabricated in real life while being fully functional and looking nearly identical (within a margin of error) to their in-game counterparts. EDIT: With the exception of perhaps the rocket launcher (that revolving tube system would be a real bitch), and the Spartan Laser.