Explosive rounds interfere with perks that proc on a precision damage kill. When usimg exp rounds you will notice that there are two damage numbers that pop up. One is the actual impact damage, the other is the explosive damage. If you get a precision hit that almost kills the enemy and the explosion finishes them off, neither firefly nor outlaw will proc. This is because the enemy died to the explosion, not the crit. You won't have this problem if you take off the explosive rounds.