The slang term “45” potentially originates from 0.45 mm bullets or the 0.45 caliber of guns. This slang is usually used in the urban setting and among rap fanatics when referring to any gun. It can be used like:
Mikey is a big fan of the 45s. He has different types in his gun room.
Gilbert carried a 45 while he was working as an officer.
While not incredibly popular today, it is still widely used as it is easy to understand and learn. Many rappers incorporate this slang in their stage names, such as “Colt 45.”