I think you got the Heavy Shotgun's damage mixed up with the Medium Shotgun...

=\ Just sayin'.

I can One-Shot Medium bodies with the Medium Shotgun, yet the Semi-Auto Burst from the Heavy Shotgun only drops their health almost half way?
Wait, what?! The Damage stats for the Heavy Shotgun does not at all reflect the actual damage it does...=|

If you line up lower on the chest and double-fire the Hjammerdeim from a few steps away, then you'll actually HIT with both blasts. If you're aiming centre-mass, the second shot will MISS. Same thing if you're too close OR too far. If you're lucky aiming lower, the second blast will be a headshot. Either way you're going to knock the target down, and have them just that little bit more vulnerable with your next shot.

Basically, use the gun right, and it'll work.

I've done that.

I had the gun right there on them, point blank.

Can you see what you missed yet?