The US Army M16 fires a 5.56 mm bullet, which is roughly equivalent to .22 calibre. I think you can call it high-powered.
Note that a high-powered rifle is not necessarily better than a low-powered weapon. One of the complaints about the M16 is it's lack of stopping power, bullets tend to go "right through" the target without mushrooming. There have been cases reported of assailants shot several times with 5.56 (.22) bullets but have kept on coming.
So why does the US Army insist on wimpy 5.56 ammo instead of beastly 7.62 mm ammo? Answer: Hump 100 rounds of M-16 ammo on a five-mile hike, versus 100 rounds of ammo from AK-47, and you'll know why.
223 & 7.62x39 aren't too awfully different in weight, however there's quite a difference between 223 & 7.61x51 NATO.
The US military is looking at going with something better than the 223/556 platform , as plenty of 'stopping power' issues are being noted with it. The 6.5 grendel and 6.8SPC round were both developed. However due to the cost to replace all 223/556 firearms and the cost of the ammo , they decided they'd go with something better, the mk262mod1.
The 22lr is one of the lowest powered rounds available to most people, and it is also available in almost every country in the world, and is extremely cheap (About $15 for a 500 round box at walmart). It can be effective against people/animals out to about 50 yards, compared to the main nato round which is .223 caliber , by 45mm long, which also has a bullet 50% heavier than a 22 (40gr vs 62gr). The 223/556 is effective out to about 500 yards (With the farthest confirmed kill being at 750 yards.)
The reason the military developed the 223 round initially was because of the fact it was a 'hyper velocity' round, which would yaw & fragment on a target if it was hit within 175 yards or so.
The 7.62x39 round is great for punching holes through barriers, however has a extremely poor bullet coefficient , which means that past 200 yards, the bullet drops like a rock , compared to a 223/556 which drops badly past 400 yards.
Furthermore , rather than develop a new round, Russia chose to go with a smaller, lighter (and lower recoil) round in the mid 70s, and came out with a 22 caliber round too (The 5.45x39). The Ukraine instead developed a much more lethal round for their AKs , the m67 round (I think that's a name) which includes a air pocket & steel core to increase penetration and yawing.
Taking notes from the US & Russia , China developed a slightly larger round (5.8x42) which is said to perform better than the current Russian 7N6 round or the current Nato M855 round (however not better than the mk262mod1).
Having said all of this , many military types say that heavier/powerful rounds are better, but cost is a issue. In addition to that, soldier training is important, a heavy kicking round in the hands of a inexperienced fighter is a recepie for disaster.
No way I'd want to be on the receiving end (although I think a bullet proof vest will actually stop a .22 - nothing larger though).
