From what I've read, (and what I can remember), it was a concept that never made it into widespread production. Essentially it is a normal cannon that decreases slightly in diameter nearer to the opening. The idea was to increase pressure as a projectile moved out of the barrel during firing, thus making a projectile with higher penetrating power or greater range. Apparently the concept did indeed work, but it was so damaging to the barrels that the need for barrel changes/repair made the concept impractical. I have a book somewhere that covers this, but I think it's in Toronto. I'll bring it with me next time I visit my brother.
I hope this helps a bit, but remember, this is from memory, so you can't quote me or hold me to any of this your honor!
