The breech plug aka cascabel seems to be a separate piece that was intended to be removed for loading or whatever. First thing I would do (this is ME now, not necessarily you) would be to grind off that ugly weld or whatever bead. It may pop off with a couple of taps with a cold chisel since it looks like they tried to weld on cast iron which does not hold unless you use the exact proper welding rod. Then try to remove the breech plug, unscrew or whatever, using penetrants, torque, impact, and if you are sure it is unloaded, heat.. Until you see how that's held in, if at all, don't even think about firing the thing.