yes you can. It will still take a solid quarter inch of setback, I'd say. Before going in this direction, measure the headspace off five once fired case (unsized) and go with the average reading you get. Then measure a go gauge, or at least compare the reading to a new case. If the headspace seems right, then doing the Ackley is easy, and maybe able to get by without a setback. The get the standard chamber again, you will need to set the shoulder back and recut the chamber. Personally, I'd do the setback anyway. 6mm Remington reamers are fairly common.
Another option for you (being as it's going to cost you about $250 anyway) is to send the rifle back to Pacnor, and have them install a new (and better) barrel chambered in whatever you'd like. Your only talking $150 more last time I looked.
gary
the .243 case is about 2.045", but the total chamber will come in at about 2.075". His step should be right at 1.56" give or take .030". You could probably get by with about .180" of setback, but I'd go with .25" just to be safe. I can't run CAD anymore because I don't have an "XP" computer anymore, so I can't exactly tell you where it all comes out. Being as the O.P. has already got 6mm dies and brass, I can see going right back to 6mm or even the Ackley. The advantage of the Ackley is that it doesn't require a barrel setback (assuming everything else is OK).
gary
