Why the 3" in 20ga. for HD ? I'd suggest you buy a couple repair sheets (1/4 sheet) of drywall from Home Depot or Lowes, go out and screw it to a couple of 2x4's, actually, cover both sides. Then stand 15 or 20' (that's feet) away and make a hole - Start with a 2 1/2 skeet load and work up...