Nope.
The chemical way that shotgun powder burns is called conflagration, its kinda like a "slow-burn" albeit it happens really quickly. Black powder is chemically different in that it "explodes"... it reacts at a much higher rate.
By removing the two plastic wads, which impede the flow of the hot gasses from the explosion, you're effectively reducing the power output considerably.
The effect you get after that, is just some under-powered hot gasses wanting to escape the barrel. With the dowel in the end, they need to push it out before they can escape, which pushes the dowel to its escape velocity. It would "blow the barrel" if the dowel were somehow wedged or lodged in the barrel, and DIDNT allow the gasses to escape. Since the barrel is 3/4", and the dowel is 5/8", it slides easily in and out. The gasses in this case will just push it out when they escape.
Tomorow's test launches will show me just how much power the gasses have. Since I removed the restrictive plastic wadding which serves to multiply the outgas force, it should be more of a loud POP, than a big BANG.
Then again, the dowel is soft wood, and would be more likely to fragment and shatter out the end, in lieu of the barrel shattering. After the glue is dry, I'll place a rubber washer at the end of the wooden dowel, then screw a metal washer against it. This should help to give slight seal against the inner barrel, catching more of the outgassing force, as well as to keep the heat away from the soft wood of the dowel.
Whatsup, you guys don't learn this stuff in school over there?
