Ditto on the kudos to Thucydides. I believe his idea is closer to where this idea is headed. Trough fired rockets would require larger venturi to generate thrust sufficient to propel the rocket. Having a mortar round that generates thrust as well as captures the force of expanding gasses wouldn't need venturis with bores as large. Aamof, the plate would better capture the force of expanding gasses.
I suspect that the walls of the mortar tube could be thinner for a given bore caliber. Thin enough that the tube could be lengthened and still use less iron or steel than a comparable Charisian mortar.
Randomiser wrote:Over on the 'future weapons' thread Thucydides mentioned the Russian 2B9 Vasilek Mortar-Gun
https://en.wikipedia.org/wiki/2B9_Vasilekwhich uses a high-low pressure system to get decent range with low recoil out of a much lighter barrel than usual. The brass disk sounds a bit like the part of the ammo which divides off the high pressure and low pressure areas. Don't understand enough to know if it would work with black powder though. Kudos to Thucydides if this turns out to be an early-model high-low pressure mortar.