If it has Airsporter stamped on the cylinder, then my guess is that it is the last Airsporter model made by BSA in UK and known as the RB2 -rotating breech model. These are lovely underlever guns and have superb styling
The direct feed of pellet into the barrel makes them an accurate rifle - with the right pellets of course
The breech opens by rotation (manually) unlike the FWB 300 series where the breech slides open on cocking (Like a HW77)
If the breech on yours slides open rather than rotates then I am puzzled
John