Rick Grimes (Andrew Lincoln) can first be seen using a 12 Gauge Mossberg 500 in Season 1 Episode 4, Vatos. It was packed in his bag of guns from the police station he dropped in Season 1 Episode 1, Days Gone Bye, that he finally goes back for in Season 1 Episode 4, Vatos. He uses this shotgun from the bag in the standoff with the Vatos and ends up giving it to their leader, Guillermo (Neil Brown Jr.) after the Vatos gang turn out to be doing a good thing. Glenn (Steven Yeun) appears to be carrying this Mossberg 500 when he first gets to the Greene family farm in Season 2 Episode 3, Save the Last One, (it's believed that there's a deleted scene from the season premiere involving returning to an overrun Vatos stronghold where its possible they retrieved it. In Season 2 Episode 7, Pretty Much Dead Already, Shane Walsh (Jon Bernthal) passes the Mossberg 500 out to Glenn from the gun bag who uses it to kill multiple walkers escaping Hershel's barn. Glenn uses the Mossberg to shoot at both human and zombie enemies in Season 2 Episode 9, Triggerfinger.
Glenn (Steven Yeun) uses the 'Persuader' version of the Mossberg 500, after taking it from Rick's gunbag in Season 1 Episode 4, Vatos. He uses it during the subsequent episodes, most notably during the siege on the camp by the walkers. In Season 1 Episode 5, Wildfire he still has it as the group makes their way to the Centre for Disease Control Headquarters (CDC). A Mossberg 500 Persuader model is used in the webisodes. Curiously it features a ghost ring rear sight and a bead front sight. It is referred to as holding eight, including one in the chamber, but in real life it could only hold six.