The information I have is from John Bowkett whose facebook page I edit. He had close ties with the BSA factory going right back. All BSA springers were unchoked. Break barrel rifles used blanks from the rifling machne that had both ends cropped off to remove any choke arising from the manufacturing process. The end that had the bore most concentric with the outer diameter was chosen for the breech end, for looks, and the muzzle counterbored to allow straight through drilling for the sights and to hide the end of the rifling should it be slightly eccentric.
When he worked on what was to become the SuperTen he experimented with barrels and chose one that was made on a large mandrel but retained the choke for best accuracy in a pcp. The springers used a smaller bore in .22" for production reasons.
I asked about the barrel length and he said that he did several conversions on Airsporters where he shortened the barrel to 10" to make ahandy little carbine. His 12ftlbs Mercury/Airsporter conversions used one of his 24 coil spring made for him by Mike Wade. The Fac 18ft/lbs version used a 32 coil spring. These were popular with certain sections of the Northern Ireland airgunning community at the time who preferred shooting British airguns.