You cannot add metal with a reamer. The 6mm Dasher is one of the many 6mm BR Improved variants, and uses 6mm BR brass as the parent cartridge. As such, the case is approximately 1/2" shorter than the 243 case. So you could use the 243 barrel, but you would have to set it back at least 1/2", and at that point you might as well just go ahead and whack the threads off completely and rethread the barrel. Once you have done that, the 6mm Dasher reamer would clean up the remains of a standard 243 chamber very nicely, and perhaps the 243 AI as well. If in doubt, cut it back 1" or 1.25" and you will clean it up for sure.
