The most thorough ballistic tests done on buckshot for defensive purposes hold that 1 buck is ideal: it's the smallest pellet that will still pass FBI gel protocols, and has the greatest total "acreage" of wound channel due to number and size of pellets.
Federal makes a superbly reviewed law enforcement load in 1 buck, but good luck finding it. I think their comparable low recoil load in 00 buck is excellent, as is Remington's Managed Recoil line.
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That (Federal #1, I think they call it "Flite Control) is in fact the right answer if one is using buck, for the reasons stated. I don't use that round solely because long before it came out I had stocked up on 00, 000, and slugs. IF I load with buck, it is generally 000, as I prefer fewer heavier pellets for more consistent assured penetration and less risk of fliers. I would not go smaller than the Federal #1 for any purpose; #4 is simply not consistently reliable against heavy jackets, etc.
However, even in my basic academy in 1989, we were taught that buck was a special use, limited purpose load that should be avoided for most purposes. We trained and qualified with slugs, and that's what I have in my M590. This is especially important if one is likely to encounter offenders in vehicles - slugs if limited to shotgun, rifle preferable. The better slugs are the Brenneke, which I could not obtain in any quantity when I stocked up on shotgun ammo 10+ years ago, so I am using the old school Foster style. Also in my academy, they had multiple reports of slug use on bipeds, and really came down hard on anyone who whined about the risk of "over penetration". It not only was not happening, but was (and is) a far less significant risk than missing.
Of course, command staff are usually 20-30 years off the curve, and I was just speaking with a friend at WSP about this. They have been fighting for years to get rid of buck and go to slugs, and even with the problems they have had with buck (fliers, etc), they can't get the executive staff levels to comprehend the technical issues. Same thing with red dot sights and mounted lights on long guns. (I've dealt with those levels of WSP before - there is no risk that any of them will ever meet the Nobel committee. I swear that the only reason they have heads is to keep their ears from touching.)
