Johnny's Magical Assault Rifle is the signiture weapon of the Spartan-II John-117, aka Johnny. The Assault Rifle is magic in two respects - first, it never seems to run out of ammo, and secondly, even if Johnny doesn't have it out with him when he's fighting, it will appear in his hand for the duration of the conversation, and then disappear again afterwards.
Johnny needs this Assault Rifle in order to display emotions, without it he is a cold menance, utterly unfazed by the deaths of his squadmates.
Jorgey's friend Noble Six had a similar weapon, but this wasn't with him all the time, only on certain occasions, usually when talking to his squadmates. It is possible that these two weapons are in fact the same, and Johnny lent his gun to Noble Six for the duration of the Reach campaign.
It is also the only Assault Rifle that can be duel-wielded, as shown before Noble Six dies.