Those were actually the mainstay of Japanese firearms. They are more like a musket with a pistol grip, actually. Often preferred by the warrior class and first introduced in a matchlock musket form by the Portuguese around 1543. The example you show here looks to be the refined form of the Japanese gunsmiths from around the mid to later 18th Century from what I can tell. Some anomolies are as follows. The barrel appears to be 17" long as opposed to the more traditional 40"bbl "average" length found on such firearms. Also the mainspring appears to be internal on your example, which while not unheard of, is not that common from what little (VERY little) I know of Japanese firearms of the era...
"If you put one in the snot locker, it doesn't matter what calibre it is".