For the information, " O " buckshot is nominally .32 caliber. "00" buckshot is nominally .33 caliber. "000" buckshot is nominally .36 caliber, but the 000 buckshot I bought from Hornady is actually .35 caliber. No. 1 Buck is .30", No. 3 buck is .25", and No. 4 Buck is .24". I am assuming that if you could find it, #2 buck would be .27 ".
These buckshot sizes make cheap ammo for small caliber rifles, and pistols, as well as serve well where buckshot can be legally used in hunting game with a smoothbore. The weight of these balls, however, is the limiting factor in the range they can be used. The .36 caliber is about 60 grains, but the .32 is down to 40 grains, the same weight as the bullet in a .22 long rifle. The #4 buck weighes in the teens, and might be effective shooting rats, and 13 stripe squirrels, at 10 feet, or so, but not much further than that. Use in a shotgun, however, so that the load is throwing a number of these smaller balls, they become lethal out to 30 yds or so. I would not worry about the effectiveness of #4 buck as a home defense load. I would worry about over penetration and danger to innocent neighbors and bystanders. That is why I use light trap loads in my shotguns for home defense, using #8 shot. (.09"). A one ounce load at the short ranges inside a house, is more than adequate to stop any intruder, without worrying about hurting someone across the street, too.
If you happen to also shoot cartridge guns, these small balls can allow you to work up " squib " loads for the guns shooting only these round balls, pressed into the mouth of the casing, with a few grains of Unique to push them out the barrel. These are subsonic loads, that allow you to practice a lot with a rifle, for only the cost of the primers, a little powder, and the cheap ball. Out to 25 yards, you should be able to find a load accurate enough to use to shoot squirrels. That extra hunting time with the gun before deer season can make you much more confident in how he gun mounts and shoots. Then you can go hunting with your ML rifle in ML season, too.