you should beab le to do it with your current model



your going to cut your mesh along the black lines ( inside front cylinder area will be cut in half as well)

the area in yellow would then be inverted ( to make a curved depression where the pin would rest)

the red area you would build a helf or full cylinder at the bottem and top ( top would have the pin and also be connection point for the bullet cylinder)

then just build in faces to fill in the inside areas.