Jeepers, a shot snake hangs nozzle down so gravity aids the dispensing of the shot. So if you just have a simple plug in the nozzle you are going to be picking up a lot of shot, or balls. If it takes two hands it kinda defeats the purpose of the whole thing. These are normally used in hunting, or quail walk, scenarios where one hand has to secure the gun.
I'm sure if this was an easy fix it would have been done in the 19th century. A cutoff system that meters single round balls is going to be pretty hard to make in a compact enough package to make a ball snake convenient.
Hungry Horse