One quick idea...

[viskaren]

I was making one of my basic combustions, muzzle loaded, and I had an epiphany of sorts. By beveling the end of the barrel, you can just load a potato that would not normally fit down the barrel. The beveled edge would cut the potato as it was loaded, and ensure a perfect seal, every time. By the way, for anyone who remembers me, I'm back .

[inonickname]

That's extremely common in spud guns. Old concept. still is a good idea though..

[rp181]

You where probably remembering it from the good ol' times =p
Its used alot.

[Xxplosive42o]

I don’t mean to jump on the band wagon but that’s pretty much what everyone does now-a-days. You create a shorter more drastic edge on the interior and a longer less dramatic knife edge on the exterior, thus ensuring a smooth easy cut that fits tight.

Cheers! 8)