That is a really good observation, your are beginning to sound like Spyder on another site.
I, personally am worried about the pressure rise between the Primer/powder and the Dowel/ball. I understand that the powder load has been lessened, but I wonder what the powder burn rate is. I know one thing for sure, a 22 caliber blank will launch an empty tin can a long way. I also know that increasing the volume of powder really makes stuff happen, I have the hearing loss to prove it.
I reload rifle rounds to bench rest tolerances, and have seen dramatic results in performance from 1/1000th headspace. The offset of the attachment of the line in relationship to the dowel will undoubtably impact the flight of the ball. Maybe he should put some arrow fletching on the dowel to stabalize the flight, I don't know how he could keep from buning up plastic fletching though?
It looks like he has adequate scales to assist in reproducing a reload, I hope nothing happens, so he can try it again. My glass is only half full on this one.
