This was an early Mercury but probably applies to others apart from the "s" which has a different breech set up and maybe more. Later ones have the articulated cocking arm similar to mk5 Meteors. Im hoping someone will put up the Bsa sheets that came with the guns to aid the thread.
Remove sights. Remove two front stock screws and large bolt that goes through pistol grip into trigger block. Remove action from stock. Remove cocking arm pin and cocking arm. Break barrel and depress barrel plunger and remove barrel pivot pin. Remove barrel,plunger and plunger spring (as always, looking outfor flying plunger!). Now you have to unscrew the trigger block. The one I did came out easily but you have a choice of oil filter straps, carefully gripping cylinder in vice and sliding a bar through that big hole in the trigger block and hitting it with a mallet. Some people have used heat and pipe freezing sprays. As alloy would contract more when cold I suppose you could always put the cylinder in the freezer for a few hours (I HAVE NOT TRIED THIS YET!). Once the block is loose unscrew it and carefully remove. Then remove guide, mainspring and piston. GO TO NEXT POST.