Flipping the script: take a cue from Spigot/Rifle grenade?

[jackssmirkingrevenge]

While during wartime it was a good idea as it saved making expensive rifled barrels, in terms of performance it's not that good a concept.

Ultimately, your projectile has a larger diameter than the surface area of the base where the gasses can push. For the best performance, the inverse of the idea is ideal, basically a saboted round, where you have maximum area inside the barrel (more pushing power available) and minimum area once the projectile leaves (reducing drag).

The only reason I could see for using this sysyem is if you want to launch an outsized load, something like the "Flying Dustbin" spigot mortar used by the British on D-Day.