I wanted to share this method of cleaning out a blocked vent hole. It is entirely possible that this method is well known, but I have tried googling it to no avail and had to invent it myself (or at least reinvent it if commonly used).
At any rate: the vent hole of my Tanegashima was completely clogged, to the point where it was sold to me as "undrilled, probably ment to be drilled open in case of expected imminent use".
Poking around the bottom and inwards with a needle revealed that the vent was not undrilled, merely filled with rust, and caked dirt. Quite hard to remove.
The needle would only work for so long, as the vent hole changed angle after a few millimeters, and then started going inward towards barrel. After having barrel for almost 2 weeks, qualified gunsmith declined to try to open. Unsure of why, I spoke only to fellow at counter of gunstore. (Wisely (I assume) he also declined to remove bisen on account of him deciding that it was just as likely to break as come loose).
Therefore I took about 2 inches off the end of a bicycle gear wire and attached it to a drill. I had to bent back a few strands to make it flexible enough to bend into and around/through the corner of the vent.
Drilling carefully, the wire slowly but surely went all the way through and into the barrel. I had to use two pieces of wire as they kind of disintegrated with prolonged use.
Having drilled all the time in the direction of the twist of the wire, I now reversed the drill, to rotate against the direction of the wire twist. The wire was fully inserted before reversing rotation. This caused wire to open up and press against inside of vent wall. This removed a lot of gunk and dirt, and vent is now completely open.