Those multiple nippled guns are very rare, cost was a factor but so was forgetting and firing the backmost charge first (or so I'd imagine)
Quite safe if one fires the front one first as BP doesn't usually ignite from compression.
Never fired one myself but have fired multiple loads in succession in a Roman Candle gun which I and the late Linton McKenzie built using a piece of seamless drawn tubing for a barrel.
Roughly thusly:
Fires VERY fast
Bye the way, Graeme, is that one from your collection??

Self Defence is not only a Right, it is an Obligation.
Eoin.