So how does a .400 brush clean anything WELL above
about .411 and especially .416 and .423 ?

And what is the difference between using a Kynoch Brush
and cutting down a 12 Gauge brush to the correct bore size ?

And as someone else pointed out, even a .411 brush
can be tight in a 404, it just depends on the maker
of the brush as they all seem to be different width's
regardless of what is on the packet.