The M133 is a pump-action shotgun that can drop most unarmored or lightly armored soldiers in one close shot. El Rubi is its unique version, built by a Yaran furniture maker.
Keep it for tight spaces and doorways. It is strong against normal soldiers up close but falls off fast at range, and armored enemies need more than one shell.
El Rubi is the M133's unique variant. Like other uniques, its mods are fixed and it cannot be changed at a workbench.
1887
The 1887 is a lever-action shotgun with a seven-shell tube that you can extend to ten. It deals solid close-range damage but works the action slowly, so it rewards careful shots over panic fire.
1887 Sawed-off
The 1887 Sawed-off is a cut-down lever-action shotgun built for hip-fire at point-blank range. FND Vanguards, Engineers and Special Forces Snipers carry it, and Rodolfo Claudia's insurgency rewards its unique Bases Loaded version.
KSG
The KSG is a semi-automatic shotgun with a 15-shell capacity, so you can keep firing in a close fight. Buy it from Juan's arms dealers, or get its unique Excavation Execution from The Long Drop treasure hunt.
RMS-18
The RMS-18 is a rank 4 shotgun with a 12-shell magazine, so you can keep firing longer than with most shotguns. You can buy it or loot it from FND chests.