I talked to a Viet Nam vet at the range a while back. He told me they used #4 or #6 shot for breeching
Hmmmm, I would think there would be a difference in "breaching" the walls of grass hooches in Vietnam compared to the steel core or heavy wood doors we have in this country.
Want a real test, go to the junk or scrap yard, find an old door matching your doors and shoot them.
Set them out at what ever distance you plan on shooting and shoot it with your buck shot, slugs, and pistol/revolver ammo.
Take you shot gun and shoot at a B27 or similar target at, 25 yards or what ever range you plan on shooting and see if you can keep all your pellets on target, same with your slugs and pistol/revolver rounds.
If nothing else, you're gonna have fun playing with different rounds at different targets seeing what they do.
I do that, (playing) but like I said, I don't use shotguns for SD, but I do shoot them (slugs, buckshot, and bird shot) in three gun. I load then so as they pattern (all pellets hit the target) at what ever range the match calls for.
What I did find is buckshot patterns better if not pushed too hard, same with slugs (more accurate) and modest velocities out of my smooth bore 18 inch 870 barrel. Regardless how hard they hit, buck shot pellets are useless if the spread prevents them from hitting the targets.
To give you an example, my slugs, If I push them at 15 - 1600 fps I hit my 8 inch popper target about 20% of the time at 50 yards. If I cut the velocity to about 1200 fps, I can hit it 80% of the time.
My homemade buckshot loads also pattern better at slower velocities.
We'll never know what our loads in our guns do via the Internet, we have to go out and test them in the same conditions we plan on shooting them.
I bet when all is said and done, If you do some honest testing on your own, you'll go back to your handguns for SD.