My response to the translation notes:
In itself, the gun Magoichi used to launch bo hiya fire rockets themselves aren't flintlock blunderbusses as in the European style. Most notable is the Japanese snap-matchlock mechanism with brass pan cover.
The Japanese made many natively-developed types of their arquebuses, this hiya zutsu is only one of many; there are large-bore handcannons, long-barreled wallguns, carbine-lengths, and even matchlock pistols.
The more fanciful weapon used in this chapter are the wheellock pistols (some are double-barrelled variant even). The mechanism itself were already known at the time (for famous instance, Willem I van Oranje was assassinated by one in 1584, this chapter most likely set in 1570); but whether this mechanism was already known in Japan is questionable. There are Japanese-style wheellock pistol specimens (which looks more like Japanese Teppo in appearance like Magoichi's hiya zutsu), but I don't know whether those really originated from 16th or 17th century. What I can say is, that the pistols in this chapter are of European style, so possibly imported or copied by some Sakai gunsmiths.
EDIT: I just realized his gun have modern grenade launcher ladder sight