The 6MM Rem has a bit more powder capacity than the 243. They both shoot the same 6MM Diameter bullet but the casings are different. I have had several of each and as a matter of opinion, I like the 6mm Remington much better. It killed deer much better for me. However the reason was the bullet not the caliber. I shot Hornady 100 grainn round nose bullets in the 6MM. These days I wouldn't use either for deer and would move up to the 6.5 Swede, 25-06, 260 or 7MM-08. This said I will be using a new 270 this year.