Mike, the barrels in the older Brummy built BSAs were a British made one, unlike the ones they use now in their PCPs (not their Spanish springers) which are I believe are Walther. I think the Supersport use's the same Spanish barrel as the Lightning on which it is based, so you're not comparing like with like.
All the older .22 British barrels were 5.6mm and not the European 5.5. There was never a specific issue with .177
Lewis.... Check your kit over, make sure all that needs to be tight is tight. I don't see a moderator in your pic, so I guess that's not an issue. Try a different sc