Hi
I have the Tun as equal to 4 Hogsheads , 6 Barrels or 2 Butts making 216 Imperial gallons.
The smaller measures being the Pin at 4 1/2 Galls, the Firkin at 9 Galls, and the Kilderkin at 18 Galls.
The type of tundish I remember using in my time as a bar/cellarman was like Pete's picture in
Post 31, but incorporated 4 legs to sit on the barrel.
Kind regards
Dave