Posted by: Rob Terror
Despite the inconsistencies, you really should take it with a grain of salt. The trailer is just to announce the new game and to show off some possible new gameplay mechanics, weapons, etc. It's not meant to be canon.
But it actually is more canon then Reach. believe me. The armor, the "jet pack", the forerunner homeworld. Everything except the magnum or whatever it is. But it might be canon too since it can be a device in cases like in the trailer where he used it to clear the path. Or maybe it's just a pistol-like grenade launcher.