I'm not responsible for your lack of arguments.
You lost sight of the visual of fire weapon.. Have a better look at the LSAC 218 Machine Pistol:
View attachment 2985344
See where the ammo magazine is. Imagine the size of the cartridge and load it into the "barrel".
Yep, the "barrel" is mostly a barrel muzzle.
The sealed acceleration time(in the cartridge) is very limited, and only a small proportion of the total energy generated by the propellant combustion will get transferred to the projectile.
It also means that, to obtain sufficient muzzle energy for the projectile, the cartridge needs the fastest-burning propellant available.. Best way to have more unburned ejecta by-products(unburnt powders, dislodged fouling).
All your barrel argumentation falls apart, and you'll always have unburned ejecta by-products, even with the best propellant/oxidizer ratio for the initial deflagration in the cartridge.
The unburned ejecta by-products are my only concern in an oxygen free environment, and also how to recreate the condition for a proper muzzle flash. I think that the "long" cartridges("which are self sufficient in terms of propellant") will provide this condition.
P.S. And you're right, end of "discussion".. Thought it was already the case with your "trollesque" reply about homage...