>>64416868
>Then the powder could be ignited by a spark plug or something.
An electric spark doesn't set off black powder or smokeless powder.

>Making cartridges and especially primers is not feasible at home
Making primers is absolutely doable at home with some basic tools and commonly available materials.
It's however very slow and tedious work, so it's only suitable for very small scale production.
And priming mixtures made from clorates are all leave extremely corrosive salt residues in the guns after firing.

Drawing cartridge cases from raw brass plate is a lot more complicated and requires very specialized tools and machines.
But non-conventional construction of cartridge cases, such as 2-part cases made of factory-made brass/copper pipe, with a separate casehead soldered on would be possible. Although it would also be slow and tedious work, and the cartridges would not be suitable for high pressure smokeless loads.