IIRC the way it used to be was .30 cal. and under was small bore, .31 to .400 was medium bore and above .400 was considered large bore.
That's still where I'm at.
Pretty much but sometimes I pick nits over the 40-43's being true big bores vs. large mediums.
As far as the pistol cartridges, that's a tough one because the 454, 475 and 500's from 6" barrels are achieving the same or higher performance as a black powder 45-70 rifle. Those 350-450 gr loads will clock upwards of 1700 fps in a rifle length barrel.
I can't imagine anyone who's fired a 458 lott would ever consider a 300 mag a big bore.
