Mobility, recoil management and capacity are unacceptable with shotguns...
Recoil on a 20ga with an 18” barrel is nothing, mobililty is manageable. If you need more than 5 better learn how to shoot. #4 Buck has about 22 pellets on the first 2, after that it’s 00. Each traveling about 1250fps or more. Works for me.
Use 1 3/4" "home defense" shotshells in a Kel-Tec KSG dual-magazine bullpup. Twenty rounds or so of No.1 buckshot should take care of it.