Hatsan do an adjustable one but that's not to say it will fit your rifle . possibly get adaptors made . The problem is the adjusting . It has to be done with the ram outside the gun . If you put it in and its 8 ft lbs .You need to remove it and pump it up then try again . If its then 13 ft lbs you have to strip it and remove air .
https://sharg.pl/product-pol-103612-...TYPE-1-ST.html
The AWT/Theoben .I had one in my AIR ARMS Hi-Power . Fitted it my self and they are non adjustable . I had to fit it my way to get the power up . They said to leave the spring guide 4mm but I left mine long for fine tuning .I adjusted the length till I got the power I wanted .
The Gamo one for your BSA is the same as the AWT/Theoben in design . They cant be be adjusted hence why I did the th fine tuning route
It was fine to shoot . I traded it not long after as my hobbies changed .
There are other gas rams made throughout the world as seen here
https://airenlaces.blogspot.co.uk/20...iston-igt.html