I agree with Gary.
Rookie, if you're still reading, look at that above post. Clearly, despite their username being a little "progressive", the shooter doesn't suck at this stuff. It takes about 98.5% shooting to assure yourself of points margin to "MAKE" High Master.
For F-T/R, that's a 1 MOA 10-ring at 300/500/600. I'd be willing to bet that 50% of those tnes were inside the 1/2 Minute X-ring as well.
If you need to shoot smaller groups than 3-5" at 600 yards, you probably do indeed need a bigger cartridge.
Bob Gill, a pretty "okay" Long Range guy, also chooses to shoot .223 at 1,000 yards. He is a bit of a masochist to be sure, but he does succeed at some of the highest levels.