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, the team of Red Shield agents led by Clara all use SPAS-12s when the group assaults the Cinq Flèches facility in Vietnam. Lewis also uses a SPAS-12 on various occasions throughout the series.