The time required for the load to travel the length of the barrel is on the order of .003 of a second. During this time the movement of the gun in ee Recoil would be on the order of 5/16". Enough inertia is imparted to the gun during this Extremely brief time to continue its travel in recoil. Brother they all give it a Fast Rap. In speaking of powders fast & slow are relative terms. All powders suite for use in a shotgun are Fast in total propellant relativity, some are just a minute bit faster than others.
I have shot mild & pleasant loads using very fast powders & some uncomfortable loads using very slow powders. it all boiled down to the weight i was moving out the barrel & how fast I moved it.
