The S10 barrel is unique among the BSA barrels in that it has the breech seal in the barrel rather than as with the other BSA guns on the pellet probe tip. An R10 (or Ultra/Scorpion/Goldstar) barrel can have the seal groove machined in the breech end to fit an S10. It's not so easy to mod an S10 barrel to fit the other BSA's as the seal groove would have to be filled or the entire breech end chopped off and re-machined (transfer port, grub screw lands and the "chamber" area re-cut).
