The only stroe-bought buckshot I have tried, was Hornady 0 buck, which measured .320". They were VERY tight getting them into the bore due to the hardness, however, once in
the bore down they went.
I bought a DC Lee mould in .319", which cast .320" round, in each cavity. They loaded easily in the .320" bore, using .022" patches, almost identically to the .311's I normally used
in the barrel. I did not need a short starter with either the cast .320's nor the .311's and demonstrated this several times on the trail. The .320" red box buckshot needed a short
starter and a good whack with the palm on the starter's knob.
It is a very fast casting session to cast up 100 .32's using Lee DC moulds.
I would think the #1 buck, at .310" would load quite easily, with a .022" patch.
This is the crown on that .32 bl. Note the wider lands than grooves, yet it still loaded easily due to the size of the balls.