Hell some guns will just kinda do it after awhile. M1 Garands do it with age relatively easily, mind you good luck with the accuracy automatic .308 was abandoned for a reason.
All you need for bumpfire to happen is hold the gun in such a manner that the recoil pushes the gun away from your finger and you pulling forward causes your finger to hit the trigger.
People use beltloops as a way to keep your finger stationary. There are plenty of YouTube videos on how to do a bumpfire.