Careful! That "something" could very well be a loaded rifle, with that stoppage being a powder column and seated ball
If you don't have a co2 discharger with a flintlock tip, designed for this very purpose, you are likely going to have to draw that puppy with a worm....and that is anything but fun.
But I would first try removing it with air-pressure...while pointed in a safe direction.
I have heard of a football / basketball valve fitting through the flash hole on some these older Miroku guns..
If you have an air compressor, and a football valve, and a little bit of luck, you may be able to move that "mass of something" out of the barred.
If you should be able to move it just a little bit forward, be sure you seat it back before trying again.
That kind of action seems to work best with co2 dischargers, but it does work.
Without doing these two relatively simple things, you may well need to taKe it to a Gunsmith
As far as the gun itself, Dixie Gun Works sold train loads of these things back in the late 60 mid 70's.
Miroku is a well respected Japanese Gun Maker when it comes to modern firearms, but many of the muzzleloaders they made, shipped to the states, and were sold by DGW just were not the quality of their other guns.
Once again, be very careful with that gun.
Russ...