The demo was never supposed to be a CQB perk, and given the massive damage that he can do at a distance the AR15+M203 combo makes him ridiculously OP. Imho if we have to have attachments what I would love to see is just cut the whole thing and replace it with a LAW with an underbarrel M203 or KAC Masterkey and a 2 shell tube magazine.
I was seriously hoping it was a masterkey when it was in the unreleased pictures where you only got an outline. However, I believe it to be quite balanced. The m4 isn't so much weak as it is just powerful enough to NOT overpower another weapon, and the launcher makes it effective at crowd clearing. The weapon by itself still can't handle the larger specimens quickly as the power on it isn't intended to kill everything.
I believe the weapon is balanced but I still think it should get commando bonuses.
3 round burst is overrated, it's more trouble than it is useful. If i was shooting a person 3 rounds will take them down, If I am shooting a crazed zed I prefer auto.