I have never measured my groups when shooting slugs and, I rarely measure other groups I have shot. At work: We shoot a ten rounds of buckshot, not for score at paper; we also shoot buckshot or slugs for tactical shotgun, not for score.
I do know that when I shoot to qualify at 25 yards, I am usually the fastest shotgun shooter in my office by far. We shoot only four rounds from the gun, combat reload with one and shoot it. That is all we shoot for qualification. My groups of 5 shots are often 4 shots touching with one flier, less often all 5 touching and, sometimes none touch. When they are all close - not necessarily touching one another, the group is less than the size of my fist. If I really take my time I can, fairly often, make em all touch within what I guess would be a three or four inch diamter circle. That is when shooting a Remington 870 with either rifle sights on a smoothbore barrel or when shooting using Ghost Rings on a smoothbore barrel - all of that is shooting without a rest from standing position. I imagine shooting from a rest, taking a bit of time, I could maybe do better.
As for the advice to have the 'whole pad' in contact, I imagine that meant the recoil pad. I rarely if ever have more than about 2 inches of the bottom of the pad/butt plate in the v of my collar bone. I cannot remember the last time I did something like try to place the recoil pad against my shoulder so the whole pad connected because that invaribly means butt to muscle mass and that hurts real bad with a shotgun. Maybe it was more than 25 years ago when I last did it on purpose. All I can say is ouch for anyone who does it that way. Just way too painful and scary for me with a combat gun. Holding it in the v formed when you raise the rear elbow is the way I go; unless of course the gun slips out doing tactical. Then who cares, I shoot to survive and a little recoil never killed anyone. Though, when I can avoid the ouch it is in the V for me.
All the best,
Glenn B
Just love that Remington 870. I shoot it quite a bit more than required at work.