Hello, I’m building a potato gun with a 4” combustion chamber and a 2” barrel. What would be a stronger transition from the 4” to 2” pipe, a bell reducer coupling or a flush hub bushing?

I built a 6’ potato gun (4” x 2’ combustion chamber and 2” x 4” barrel). I’m using aqua net hairspray for the propellant. When shooting, a puff of white smoke comes out the end cap that is screwed on hand tight. The igniter is screwed into the cap. There is also burn marks on some of the threads. Co...