With a 4" rifled bbl, the shot pattern opens up FAST. It'd hard to get more that 10 or 15 pellets on a pop can at 15 feet with that short of bbl.
That's the truth. I use #9 in my .44 mag homemade shotshells. Beyond 15'-20' it spreads out too thin to be of any use. One of these days when I get some time I'm going to try varying the powder charge to see if that will tighten it up a bit.