I don't know of any use of shotguns by formed units since WW I. But they sure used them then. If you like the shotgun, you will appreciate this report by Jan Stevenson:
"Trench raiding was what the gun was intended for, hence the name. But it could be quite devastating against the massed infantry attacks characteristic of the Great War. Paul Jenkins recalled one incident when the '97s were used to give concentrated fire across a section of front after the rifles and machineguns had done their best."
"When those shotguns got going - with nine .34 calibre buckshot per load, six loads in a gun, 200-odd men firing, plenty more shells at hand - the front ranks of the assault simply piled up on top of one another in one awful heap of buckshot-drilled men."