So far, I haven't seen a really solid definition for "high-powered rifle."
Me either.
I have taking to telling my gun illiterate friends, acquaintances, and Facebook antagonists that they are gun illiterate, and they don't get to tell me what "High powered", "Intermediate Powered" or "Low Powered" cartridges are. I'm the gun guy, I actually load these cartridges, I actually shoot them, I actually modify guns designed to hold them and am familiar with the pressures and forces, so MY opinion is what counts. Basically "I don't tell you how to cook Kale or make a tofurky, you don't get to tell me about firearms."
I did have one young college female student (Daughter of one of the Mrs' coworkers) argue that one didn't need to be an expert to talk about things. So I told her to stop trying to lib-splain over me, it was insulting and a micro-aggression. She got three posts and about 1200 words into explaining that as a white male I didn't understand micro-aggression and wasn't qualified to point on out to her, the woke feminist, and that I sounded like an idiot to anyone that knew anything on the subject before she realized.
FWIW, and roughly*, for rifles:
Low power-Rimfire or Pistol cartridge in a rifle.
Medium or intermediate- anything centerfire under about 2000 ft/lbs at the muzzle. 7.62x39, 5.56, .30-30, .300 BLK and the like. The "Assault Rifle" calibers
High Powered- more than medium. .308 and up.
*Mandatory gun forum disclaimer: Obviously there are a TON of cartridges and loadings. above are rough numbers for educating non-gunnies that want to learn. If you're about to bring up a something like .276 Pederson, anything any Arisaka was chambered in, or a wildcat that was in one article in the 1959 issue of Guns and Ammo, these categories aren't for you.