The M21 has dovetailed Chopper lump barrels for one reason only, and it is not because they are stronger than brazed lumps. The decision was made to build the gun with heat treated steel barrels, and that precluded brazing on the lump, because brazing heat would have annealed the barrels. The only method that could be used was dovetailed chopper lumps with soft solder for attaching ribs and lugs.