My rifle chambered in .17 Remington fires a 25 grain bullet with a muzzle velocity just over 4,000 ft/s. Now, my AR-15 is chambered for 7.62x39mm (It’s a long story), but I doubt that it launches one of the 123 grain bullets in the Hornady ammo I use for deer hunting at 20,000 ft/s. That would result in a muzzle energy over 100,000 ft•lb. which about 8 X the muzzle energy of a round fired from a .50 cal. M2 machine gun. My AR doesn’t seem to kick that hard.
UPDATE—FWIW, the typical muzzle velocity of a bullet fired from an AR-15 chambered for common 5.56x45mm ammunition is about 3,150 ft/s. The only combination of firearms and ammunition that I can find in my gun safe and ammo locker that could produce a muzzle velocity as low as 625 ft/s (1/5 of 3,150) would be a short-barreled revolver loaded with .22 Shorts or a muzzle-loading rifle with a dangerously low powder charge.