Pack gunpowder into the back of a tube, optional washing between, then some projective from forms to bb’s. Aim, fire. That’s all it takes close range.
You forgot the primer or ignition system. I am wondering if the guy didn’t use shells but went completely home-made, and if so, did he skip the primer stage and go for electric/electronic ignition.
Japan has ammo and gunpowder control as well.
Sort of like an old blunderbus from the 1400s 1500s
“Pack gunpowder into the back of a tube…”
I think you’re channeling Jim Kirk fighting the Gorn.