Bill is correct that weight and velocity determine the energy which goes back against your shoulder, but that amount of energy delivered to your shoulder can be delivered as a "punch" (hot powders) or as a "Push" (slow powders). It has been my experience that for a given velocity, you need more of a slow burning powder than a fast powder. I suspect that promotional shells ("Wally World" as an example) have small amounts of very hot powders. Several implications - costs less to produce those shells, more of a "Punch", and for some people, the perception that those shells are hard hitting, and therefore "good" shells.