Almost no one owns an m16. That would be a NFA (National Firearms Act) weapon that requires a special tax stamp for and would also have to be made before 1986. No new fully automatic firearms have been allowed to be made except for LEO/military purposes since 1986. All the of what you're calling an "m16" is an AR-15 (Armalite Rifle 15) or some variant. They're all semi-automatic.

The AR-15 is so popular because it is a very accurate, reliable rifle, and can be very affordable. It also helps that it's the civilian model of the m16 and m4 rifles the military has. I have one that I target shoot with and it's a very fun rifle to use. That's probably the biggest reason it's so popular.

I do agree that we need much stricter gun control in this country but it needs to be based around logic and fact. The "assault rifle bans" of the 90s were completely useless and didn't van anything meaningfully dangerous. They banned things like collapsable stocks, threaded muzzles and other silly things that don't effect the operation of the gun. Hell, California currently bans rifles with pistol grips, so when you buy an at there it just has a different grip. The function of the rifle is still the same. Personally I think we need stricter licensing on who can own a firearm. You should have to prove you are competent, capable, and safe in your use of firearms before you're allowed to own one. That includes VERY thorough background searches and atleast 20-30 hours of instruction. The old tired adage of "guns don't kill people, people kill people" does have some truth to it. But if less people had the guns to begin with, they'd have a lot harder time hurting people.