The propellant cartridge placed into the tail body assembly of a 4,2 inch Mortar shell (bomb) exerts its Gas pressure through the Paper part of the Shell case, and then through the Holes in the steel Tail shank. The End cap is to prevent the Case Crimp from opening up, and thus allowing all the gas from the cartridge to be vented into the “Low pressure” space around the Tail assembly, to propel the Bomb out of the Tube and onto the Target.
This gives the Gentle" Push" rather than a Fuze-jarring “punch” of the charge going out the crimped end of the shell.
4,2" Bombs usually had accessory “charges” wrapped around the central Tube, to give variable ranges, so the central shotshell charge was also known as an “Igniter” to the wrap-around charges.
During WW II, the 4,2 Inch Mortar was the heaviest Allied Mortar in use, and used for both HE and “Chemical” use (Smoke, Flares, Propaganda leaflets, etc. and even Noxious gas if required.
3" mortars also used the 12 ga. shell, whilst the 2" used a 28 gauge shell.