I totally disagreed with power of some underbarrel attachments.
The Gauss - very powerful , destroys Ceph Air Unit with 6 shots (if you don't have JAW, don't carry because of weight),
Otherwise two shot of this attachment, kills anyone even in Armor mode (Gauss bullets cuts off power to zero). I use it a lot, it is like portable sniper for me when I don't have DSG-1 or Gauss Rifle. I use assault scope to get headshots. I believe it is matter of skill to use this attachment, to be frank, it does take a lot of skill and patience to get kills with this attachment. so far I got 150 kills with it

The Grenade Launcher - Very powerful, one grenade only

but direct hit is sure kill or heavy damage within 2m blast radius. Very useful when playing on Crash-Site, very fast shot, can take enemies out with surprise. Personally not, my favorite because of only one round.
Shotgun - This is definitely a bit too weak or lags, don't know which one. I don't use because I carry a Marshall instead. I need the feel of huge ass gun shove into my opponent face XD