Explosive Rounds suck. Firefly is the only good explosive perk. Your explosive rounds cut your crit damage by a lot. Just using example numbers here: If you normally body shot for 200 damage without explosive rounds, you will instead body shot for 100 damage with explosive rounds. The explosion will give you the extra 100 damage you would have had (it looks like two numbers when you hit). When you crit, you still only do 100 explosive damage, but only your non-explosive damage crits. So WITH explosive rounds you crit for 200, explosive for 100. Without explosive rounds you crit for 400, netting 100 extra damage.