Think this has been fixed (Destiny)

by CruelLEGACEY , Toronto, Monday, April 06, 2015, 14:03 (3310 days ago) @ ChrisTheeCrappy

Basically as Funk mentioned somewhere else, the total damage is not increased, as the explosions just add back up to a normal head shot.

I have no idea if this is unique to Fatebringer, but a crit hit from fatebringer with explosive rounds does more damage to Vex than a crit without explosive rounds. Just against Vex though. We discovered this a while back when Cody said Hawkmoon was the only hand cannon that could kill a level 30 goblin with 1 shot. Speedracer said "No, I do it with fatebringer all the time". A bunch of us tried it, and none of us could do it until we realized Speedracer had explosive rounds activated. We all did the same, and sure enough we were all suddenly able to 1-shot the level 30 goblins.

No idea if this little hidden perk is inherent to all weapons with explosive rounds, though. Might just be Fatebringer.