Sarge: Sarge begins to carry it around mid-Season 1. Sarge uses it because he claims long range weapons are "unmanly." He has however been seen to use an assault rifle from time to time while fighting Blues. He killed several Wyoming clones with it at the end of Season 5. In the Zombie PSA, Sarge says he carries a shotgun at all times in case of a zombie attack.
Grif: Grif also used Sarge's shotgun in an attempt to revive him after he was shot in the head, by hitting him in the chest with it during Season 1.
Simmons: Simmons used one in Season 1 to guard Tex after her capture, and after Sarge's apparent death in Season 5 so he could be the leader of the Reds.
Church: In the flashback of Tex's assault on Sidewinder, Church had a shotgun and tried to shoot Tex multiple times.
Epsilon-Tex: In This One Goes to Eleven, Tex takes Sarge's shotgun from him and attempts to shoot Grif in the face, only to find that there was no ammo left. She then swings the shotgun right into Grif's balls, causing Grif more pain and sending him flying into Simmons.
Locus: In Worst Laid Plans, in order to prevent others from learning of the crash site, Locus killed the Dropship Pilot with a shotgun and pistol after the latter lowered his guard. He later uses it to kill Jason Cunningham in Season 12.
Shotgun is also a running gag in the series that relates to a character getting into a Warthog first. This usually involves Grif and Simmons, where one would say, "Shotgun!" and the other right after would say, "Shotgun, fuck!" after missing their opportunity. This gag also appears in Halo 3, being said by Marine Corps marines which substitute the word 'fuck' with 'damn' during the campaign level Tsavo Highway, while the IWHBYD skull is active.
As shown in Revelation, "Shotgun" is used as Sarge's code-word for a contingency plan. Sarge first allows himself to be subdued by an enemy, then makes a statement about his shotgun, emphasizing the word to signal his squad to use the Warthog to take out the enemy. Grif has expressed distaste at this plan. saying "How about next time we use a code-word, we choose something you don't say every five seconds?"
Another one of Sarge's contingency plans is "shotgun to the face" which Sarge claims is useful in a "variety of situations," from infiltration to insubordination.