In the Anime
When Saya, Hagi and David go to rescue George, David equips himself with a Franchi SPAS-12 with no stock and loaded with slug rounds. Lewis mentions that this weapon is not enough to kill a Chiropteran, but David comments that it's all he needs because Saya is the ultimate weapon against Chiropterans.
In Episode 12, Red Shield agents all carry SPAS-12s when the group assaults the Cinq Fleche facility in Vietnam. Lewis also uses a SPAS-12s on various occasions throughout the series.