I'm in agreement with all that recommended .223!
"Slightly" biased as I own 7 of them, 2 Bolts and 5 AR15s. I'd no matter what get a 1:8 twist IOT stabilize at least 77s.
I've got my own range and reload for multiple rifles, but the .223 is positively the caliber I shoot the most. I've set up my bolt rifles as "trainers", one for a standard hunting sporter, the other as a precision-ish rig. I really like practicing with .223s due to cost, lack of recoil, and the challenge reading conditions at 300-600 yds. Practicing feild conditions/rests:
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The other thing I'd recommend is a quality .22LR. Simple shots from 100-200 with a .223 or larger become way more challenging with a .22LR.
As the others also said, IF hunting is on the menu, I'd "up-gun" to either the 6.5C or 7-08, even .308Win. I've gone through a couple .243 barrels, it's a great deer hunting cartridge, but not a great round for extensive practice/shooting unless you're willing to put a new tube on periodically.