After reading all sorts of opinions about assault rifles amd the 5.56mm AR-15, but few facts, I'll introduce a little history.
The high velocity . 22's like the . 223/5.56mm began in the 1920's with the . 22 Hornet, developed by the Springfield Armory and introduced as a commercal Winchester cartridge in 1930. The round was chambered in their model 54 bolt action rifle and very rapdily became a favorite for varmint hunters, beginning a whole series of high velocity . 22's, eventually culminating in the . 220 Swift in 1935, with a 4140fps muzzle velocity. Among these cartidges are the . 222 Remington, a huge favorite, the . 223 and the . 22-250, a wildcat converted from a Savage . 250-300 and made into a commerical Reminton round in 1967, currently the most powerful of the high velocity . 22s.
The . 223 Remington, aka 5.54mm Nato, was developed by Armalite for the Army in the early 60's, then chambered in the commercial AR-15 semi-auto magazine fed rifle, labeled the M-16 by the Army for use in Viet Nam. The AR-15 is now considered "America's Rifle" because of its popularity.
Since their inception in the 1920's the HV . 22's have been made, sold and used as varminters, designed with flat trajectories for shooting at long range on scope sighted rifles where damage to the varmint is irrelevant. Throughtout the early 20thC magazine articles argued the merits of these cartridges for hunting 'Chucks, aka groundogs, prarie dogs, crows and small animals up to coyotes, providing much entertainment for hunters, emphasizing tack driving accuracy, often with custom made rifles.
For many years, starting in the late 60's thru the 80's I owned a custom made . 22-250 and hunted and killed dozens of 'Chucks, aka groundhogs, at distances up to 200 yards.
On examination of the dead animals (I never wounded one), the results were similar. The bullets exploded inside the animal and turned a volume about the size of your fist into jello, literally. Many surgeons have confirmed similar damage to the organs of victims of these exploding bullets, noting that the total destruction of the victims organ was inevitably fatal. Fired from a semi-auto rifle like an AR-15 with a high capacity magazine, many people can be killled in a very few minutes.
Because military style weapons have always been popular with the public, the AR-15/M16 has followed suit, and has been widely sold, albeit at high prices. The . 223 ammo is widely available. But note that the commercial ammo is specifically designed as a varmint cartridge, and as such is designed to explode inside an animal, due to its high velocity and construction. The bullet is a lead core hollow point with the tip closed, encased in a thin copper jacket to prevent barrel fouling that would occur if the bullet was just lead. However, the bullet is fragile.
The physical equation for the energy expended by an object is: energy=mass x velocity squared. Double the velocity and there will be four times the energy expended- applied to cars as well as bullets. That's why the HV . 22s are so lethal. Not legal anywhere for game animals because of their destructiveness, they were originally designated illegal by the Geneva Convention. The fact that varmint hunting is a "one shot one kill" activity because a miss means the animal will be long gone before a second shot can be gotten off there is no practical reason for civilians owning semi-automatic magazine fed AR-15's except to feed a macho ego and make profits for the gun manufacturers. Add a do-nothing Congress owned by the NRA, living in a gerrymandered district and you have all the ingredients for mass murder- and that is what we have.
My dad had a 22-250 when I was growing up and he used it to shoot ground hogs that were eating my granddad's crops. When I got old enough he taught me how to shoot it. If you put a milk jug 1/2 full of water and shoot it with a high velocity rifle it will literally explode. He was captain of a rifle team when he was in the reserves and taught me a neat trick. You can't hold a gun in one place, it's always going to move around a little bit, so what you do is every time it goes by the center of the target you squeeze the trigger a little bit until it fires. It works too.
I inherited a 22 rifle after HS. I remember shooting at a friends house at a bale of hay. Afterward we saw the bullet went through the hay and two layers of wood. It was at the back of my friends garage an we were relieved his dad's car was not there.
I knew nothing about guns but a workmates said it had a low serial number and was valuable. He traded a M-16 for the gun. When I went into the military I gave the gun away.