Handcanons get destroyed by scout rifles. My auto rifle does roughly 113dmg for a headshot on most normal enemies, scout rifle does 245dmg, sniper 1200+. I am testing out the scout/fusion/rocket launcher combo today as I just got a lv8 fusion rifle last night. The rocket launcher is invaluable against boss and sub boss characters to me. I really like the auto rifle too but I do enjoy taking a steady aim approach for weak points with the scout. I carry both so if I need sustained damage I can swap to my auto rifle.
As you said the shotguns aren't that great. Maybe in crucible up close, but in PvE I find other options to be superior.
I never got a good machine gun. If I do I will try it out.