I can't really offer any words of wisdom or tips on technique as I would never shoot a heavy double or actually a heavy anything at 100yds offhand with irons. In fact I would be hardpressed to comfortably take a shot at 50yds offhand with irons. At 25yds with irons I can put everything within a 3" red target spot. I wear trifocals and just plain cannot see well enough to do better. Is this good enough, I don't really know, I don't really own irons so can't say. All my big bores are bolt guns and all have scopes, the irons on them are set at 25yds, but I've never really considered using them and have never practiced with them. Only all Irons gun I have owned was my 470NE and I took one Elephant with it within 25yds. I hit it where I aimed and it died as it should have. Next one was with a scoped 416Rigby from about the same distancce with the same results. I wish you good success and good hunting.