That's not flip flopping. I didn't say I'd take a deer at close to 800 yards with a .243. Or anything for that matter.
I'm commenting on your grasp of ballistics. Your math is way off on the drop, and then you claim that a few MOA is somehow impossible to dial in for some reason. Under 8 MOA is nothing. For example, a Vortex Razor 3-9x40 has 90 MOA built into it. If everything is fairly straight and level, your zero should fall somewhere in the middlish, leaving you probably 40 MOA of elevation adjustment left. That converts to 320" at 800 yards.
You're kinda out of your element arguing with guys on what a rifle can or can't do, when there's definitely a disparity in knowledge. You have yet to state your arbitrary parameters but to say a .243 is too small, and a .300 Mag is too big. Completely ignoring that there's probably a dozen different .30 cal magnums, and a hundred different bullets to pick from! If you think the .243 is marginal at X range, just add 300 yards for a .300 WM and it'll be completely marginal too!