> If not energy, by what metric could the latter be described as "high powered"?
Well the Browning Hi-Power was so named for it's large magazine capacity. But I agree with the substance of what you're saying. 5.56 is not a "high power" rifle if that term is meant to distinguish it from other sorts of rifles. For magazine capacity, you can find 22lr magazines with larger capacities than your typical AR mag, but calling one of those high powered would be a bit silly.
The technical term for 5.56 and similar rounds is intermediate cartridge and includes 7.62×39mm, 5.56×45mm NATO, 5.45×39mm & 7.62x35mm among other cartridges. The definite from wiki is 'An intermediate cartridge is a rifle/carbine cartridge that is shorter than typical full-power battle rifle cartridges, but still has greater length than pistol/personal defense weapon cartridges.'
Well the Browning Hi-Power was so named for it's large magazine capacity. But I agree with the substance of what you're saying. 5.56 is not a "high power" rifle if that term is meant to distinguish it from other sorts of rifles. For magazine capacity, you can find 22lr magazines with larger capacities than your typical AR mag, but calling one of those high powered would be a bit silly.