Hi,
I personally swear by the 2.5 litre bowl for pigs, it is ideal for piglets, weaners, and adults and we have sold 100's of these.
It is designed for low pressure systems - so will work ok off a water barrel (as long as the water is clean with no solids to block the valve)
The end of the valve on these is brass and threaded 1/2" BSP - so any suitable 1/2" plumbing connection can be used.
Some points to note:
1) Keep visible pipework as short as possible - the pigs may try to chew it if they get bored
2) Use fasteners and washers that appear far too big to attach to a wall or posts - the drinkers are incredibly strong, but if a full grown boar stands in one it may rip out of its fastenings if you dont.
At £22.50 they dont cost much more than a hoselock connector !
Nipples are better suited to a clean mains supply for use indoors.
Thanks