I've been swapping barrels around for years, normally I put the shorter Lightning barrels on Mercs and Challengers as I love a carbine, I have had a couple of problems in the past with the mounting holes being slightly out of place which caused one Merc to not push the piston back far enough (that's what I think anyway) as it would just catch the sears sometimes and fire itself, the only real difference is the slot that is on the underside can differ in width so you may need to pack the cocking links out or narrow them, depending on the size, the only other BSA barrel design that I've found that causes problem is the ones fitted to the Brocock Independant, which although they are the same fitment are actually about half inch longer (the breech bloke that is) and causes problems with the cocking lever being too far forward if fitting to a BSA (or too short if a BSA barrel is fitted to the Brocock).

Pete