Battle Rifle ammo glitch
From Halopedia, the Halo wiki
If you are firing a Battle Rifle burst, and do another quick action (like doing a melee attack or throwing a grenade), there's a chance only one or two bullets will be fired. For example, if you fire BR bursts until 24 bullets are left and then fire of a burst while quickly throwing a grenade, you may end up with 22 or 23 bullets.
This glitch occurs simply because the Battle Rifle cannot be fired while you perform certain actions, so if you perform one of these actions while the BR is firing a burst, the burst will not complete.