Oh boy, I’m sure I’ll get all kinds of flamed for this, but here it goes: I wouldn’t say that 6cm and 243win shoot the same bullets. I’d say they shoot the same diameter bullets, and in some (many) cases are capable of shooting the exact same bullets. But in factory form what they are capable of shooting best is different. Don’t believe me? I guess ask Hornady then. Why do they offer a 6mm 80gr gmx and a 90gr gmx. The 90 has a much better bc, it also says minimum of 1 in 8 twist required. How about the 90gr eldx and the 103gr eldx, same story. Based on that I would say they do, or are designed to, shoot different bullets. Or perhaps different bullets are designed to be fired from each cartridge? I know I know, it doesn’t say anywhere on the box of projectiles when you purchase them that they MUST ONLY be loaded in xyz cartridge, but you can see the implications here. Also, see factory loaded ammo for each cartridge, kinda reinforces what I’m saying here.
I like both cartridges (have owned both), makes sense, they’re very similar. But 243 does tap out with heavy long bullets, due to most factory twist rates. Granted, Ruger and garbageton do have decent factory twist rates, savage is okay and browning / Winchester are terrible. So for most factory rifles the 6cm is going to do better with (as in stabilizing) long heavy high bc bullets than 243win.
Nothing wrong with a custom fast twist 243win barrel either, evens the playing field in terms of bullet stabilization. Cartridge oal and max magazine length can become an issue with some actions though. Could always AI the 243 for a little extra power room.