in factory loads you wont see any difference, but there are two things you need to really get the most out of the 6mm. you need to handload, and the 6mm can utilize a 26 inch barrel to get more than the listed velocity (100fps) difference. the 243 wont gain as much as the 6mm w/ the 26 inch barrel. I have had several of each and really preferr the 6mm over the 243 , I still have several 6mm's, but haven't kept a single 243. If your lefty try the ruger #1's.