To clear some apparent confusion. The BSA in question definitely has dovetailed CHOPPER LUMP barrels. I cannot discern even with a powerful lens whether the joint is solder or braze.
Looking at the construction I can see why the barrels are wider than "regular" chopper lumps. In the usual chopper lump joining metal is removed from the inner surfaces of the barrels, the lump sides, to bring them closer together. The braze is strong enough to form the middle layer into one solid wall which can be no thicker than the external barrel walls.
In the BSA-Mode 21 setup no metal is removed from the inner/middle layer. The same applies to monobloc barrels. The barrels must be totally round when inserted into the monobloc there is not chance to file the middle part to bring them closer together.
You have to wonder then whether the insistence on chopper lump and the dovetailed lump (the one inserted under the barrels) was to keep the gun slim and elegant, rather than stronger.
Personally I would rather opt for barrels that are a millimter or two wider at the breech ends and stronger.
