thats an rpg... I think all the previous users of it died because they don't have the concept of backblast in the other world so they all died to backblast.
...even tho the design of it doesnt have a back blast vent opening... I guess artist-san isnt a gun nerd
That's not an RPG, that's a
Pile bunker. Similar idea in that an explosive force causes a blast, but instead of a projectile it pushes a captive, hardened stake out to pierce armor. So in addition to the escaping gasses, you have the force of the stake shooting forward until it hits the stop that keeps it captive. Use it wrong and you could be shot backwards with most of that explosive force, or forwards as the stake desperately wants to keep going.
You don't need a backblast vent, you can do something similar with something very much like bullet cartridges and shotgun shells. The important part is the extremely sudden punch.