Consistancy of ammo is a big thing. I've used marbles and various sizes of ball bearings, but I like #1 buckshot the best. Buy it in the big bag, shoot into something like a large shipping box filled with something like rags or wadded newspaper so can recover your shot, and practice.
It takes, IMHO, a lot more practice than any other ranged weapon other than thrown knives (which are worthless but for sport) and true slings (to make a good slinger, start by training his great-great-grandfather, and make them a family of shepards).
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-IronRaven
When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.