Ammo can be replenished by Engineer's Ammunition Package Ability
The Underbarrel Shotgun is a rail attachment that can be certed for some select-fire weapons. It can fire a single shot before having to reload, and can have up to 4 rounds (1 in the chamber, 3 in reserve). This cannot be expanded using the Ammunition Belt or Grenade Bandolier.
The Underbarrel Shotgun can kill any enemy (except for MAXes and Heavy Assaults with their shields raised) in one hit at close range. This puts it on the same level of effectiveness as a pump-action shotgun. It works quite well with the Light Assault class.
To buy the shotgun, you must have the standard faction carbine with the S in its name: Gauss Compact S for the NC, TRAC-5 S for the TR, Solstice SF for the VS, or the standard faction assault rifle with the S in its name: Gauss Rifle S for the NC, T1S Cycler for the TR, Equinox VE2 for the VS (note the Equinox does not have an S). It is found in the rail slot. An underbarrel shotgun is also permanently attached to the NC's Directives shotgun, The Brawler.
All Directives Carbines have an underbarrel shotgun permanently attached.