I was doing some testing for my upcoming explosives character and I noticed that I was not doing any extra damage with this perk on certain weapons. I boosted Issac's health at the Gun Runners to a 1000 points and noticed that I was not doing any extra damage with Demo Expert Rank 3 with the Missile Launcher. (Using the perk Living Antamoy check damage) Baffled, I opened up the GECK and checked what weapons are supposed to work with this perk. Sure enough, the missile launcher was not on the list! Also, the Fatman, the Unique Missile Launcher, C4 and even Dynamite do not work with this perk! It does work with all the other mines, grenades, grenade launcher and machine gun grenade launcher.
So, any explosives characters be warned! There are bugs loose.
Edit: Added the Missile Launcher to the list and damage ramped up as expected. Using HE rounds with Demo Expert 3 and a quick mod boosted the damage up from 251 a shot to 568.