No picture .... Remove action from stock. REAR of lower tube sits a BLACK cover over the HST adjuster, it must be UNSCREWED Counter clockwise. Under that you will fine a very small & fine thread adjusting thimble with a slot. ONLY USE a square full width GUNSMITH type screw drive blade in the slot or you will screw it up !!!!!!!!!!!!!!!!!!
Sometime the factory has used a thread locker on the CAP and the ADJUSTER thimble .... heat can help break them loose.
UNDERSTAND that the BSA's mostly set up a power TAMPER PROOF gun not really being designed for end users to fiddle with .... if pieces don't turn .. DON'T FORCE THEM !