Which gun specifically from Halo? There are lots of different guns in Halo, most of which would be pretty intimidating compared to anything we have...
http://www.digitalsomething.com/media/images/halo3-1.jpg
http://www.hizone.info/data/2005/04/22/images/fn-2000.jpg
Some differences but the FN is modular so it can probably be made to look more alike.
