My Mag Tech shells take a #300 pistol primer. When I bought these I had to use a reamer bit and very lightly ream the sharp edges off the primer holes. I did this very easily and lightly by hand and it wasn't difficult. I use 14ga over powder cards and cushion wads and 13ga over shot cards, all Circle Fly. This works pretty well for me, but in all honesty I can get better, more consistant patterns using Ballistic Products gas seals and plastic shot cups out of my 16ga flintlock fowling pieces.