Many posters here are entirely correct in that the state may specify what a security guard may carry. Don't know if they still do but here in NY, armored car personnel also carried pump action shotguns although I don't know of a single case in which one was ever deployed.
Homeowners and store owners can have access to pretty much anything that's legal for them to possess in their state or city of residence. In NYC, semi-auto shotguns are prohibited and all long guns must be registered. Stay within the law, whether you agree with it or not, and you eliminate one avenue of legal attack against you. And, please don't be one of those hayseeds who loads his shotshells with rock salt.
Int'l Assoc. of Law Enforcement Firearms Instructors