MyT,
Yes you could do it, but the problem you will have is that the .243 chamber will not clean up completly and you will end up with a small ring in the chamber where the shoulder of the .243 was. I thought about this one myself and talked to a gunsmith that checked the chamber drawings and dimensions several times before we decided it could not be done. You could do a 6mm-284, but then there is the issue of the rebated rim and the extra preasure that is border line on what the Handi could take. You might get away with a 6mm-06 Improved as it would clean up barely and not have the ring. If I was going to do it I would contact Wayne York, he did a .223 to .220 swift conversion for me on my Handi. He can give you all the info you would need to decide.
Greg
