Ottoman bulbous barel wall musket

[eftihis]

Here is a 17th century (approx 1670) ottoman long wall gun (1,80cm length).This type of bulbous barell was first used in matchlocks, and this barel must be re-used from 16th century. Question is why these barels have this bulbous end? And how this gun was used? There is a strange surface on the down side as if it was attached to something else and may be used as a swivell gun.