Here is a cool video of one firing:
This type of firearm has its roots in the hook guns of the late medieval to early modern era. In the 30 years war era (and before), oversized musket would have hooks at the front to stabilize themselves on ramparts and provide longer range fire power. They could almost be viewed as the anti material rifle of their day.
German examples:
Double Hackbut on Carriage Mount
German example in German (You can see the similarity to the muskets of the era):
Doppelhaken, Suhl um 1630
Guns of this type would remain popular in India and China through the 19th century.
For the flintlock versions, the Rifle Shoppe sells kits, if you are really itching for your own.