There’s a local TV news film clip at the Fox News link, but other than a metal barrel, it doesn’t give much info on how he did it. Obviously, they don’t want that released, but I’m sure “it’s already out there” on how to do it.
Seems to me that it’s take a lot more metal parts than just a barrel, wouldn’t it?
Zip guns are pretty simple. There is no action per se just the pipe barrel and something to function as a firing pin.