Being smaller, 3 pellets of 00 will lay down flat in a 12 guage shell.
You'll typically see 2 3/4" OO buckshot rounds with 3 layers of 3 pellets (9 total) in the case.
A OOO pellet, being bigger, won't really 'fit' as well in a 12 guage case.
However they do lay down just right in a 10 guage shell, If I remember correctly-
the 10 guage has 4 stacks of three 'OOO' pellets (12 total).
And, a single stack of 3 'OOO" pellets fit perfect for the buckshot load in a .410 shell.
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