What you have is a standard shape tanegashima or arquebus. This gun was introduced into Japan around 1543 by the Portuguese. Almost all remaining examples are from the 1800's as is yours.
There is a lot of information about them on the web if you know where to look.
Check out the Wikipedia definition for arquebus:
Arquebus - Wikipedia, the free encyclopedia
Also look at the Wikipedia definition for tanegashima:
Tanegashima - Wikipedia, the free encyclopedia
Finally if you do a google search under "arquebus" you will find some interesting articles.
The retail value of yours which appears to be in pretty good condition is about $1,200-$1,500.00.