If you aren’t up for building a prop from scratch, you can always try for the modification route. Redditor Jimsbeanz did that for a Halo assault rifle. BoomCo makes a replica of the weapon for the game, and though the colors aren’t quite a screen accurate match, the gun can be painted over. Jimsbeanz gave it a complete makeover with a Halo palette and went above and beyond to add a working ammo counter. You can see the ammo counter in action over at Imgur.