Zip guns are pretty simple. There is no action per se just the pipe barrel and something to function as a firing pin.
In the boonies in Pokot-Karamoja region along the Kenya-Uganda border, the local tribes used to steal the 1/2” galvanized water pipe used as pump rods on the deep well hand pumps. They could convert these into gun barrels.
So, everyone switched to using 3/4” pipe; ain’t no one man enough to fire a .75 Cal shoulder weapon!