Automatic guns became regulated in the 1930s, and you need approval by the ATF to own them and they need to be registered. They're in a similar category as bombs, rocket launchers, and whatnot. They are strictly regulated and the government basically knows where each one is at all times.
In 1986, they stopped accepting new registrations for automatic guns. So there's a fixed supply of legal automatic firearms that normal people can own.
I'd say it's a hobby. Bullets are expensive in quantity. A full automatic can chew through $100 worth in just a minute. There are a few gun ranges near me that keep them available to rent. You can relive your A-Team / CoD dreams for a few hundred dollars.
Outside of the military they have no utility whatsoever.
Full auto on an M16/AR15 will empty a 30 round magazine in a touch over 3 seconds. A few ranges near me will rent you a full auto AR but you have buy the ammunition from them.
The primary benefit of high rate of fire is hitting a single moving target. Machine guns used to be particularly common in air to air combat. When you are a school shooter or bank robber you actually don't want to waste your ammo like that. The downsides of reloading are by far stacked against you. A soldier can just go back to base and reload his guns but a robber whose magazines have run out will just see his targets flee and his only opportunity slip away.
Just a nit pick: the government only knows where each item is so long as the owner has reported it moving around, as required per ATF laws. Wanted to mention this because someone may imagine there is a GPS attached to lower receiver of FA. If you travel with your fully automatic you need to report it to your local sheriff office (stating purpose as well); also when you move to a different city/county/state.
Another nit pick: the approval is not some sort of interview or questionnaire as of why you need it. There is so-called ATF stamp (i.e. $200 for silencer) than you need to submit altogether with FBI fingerprints. These days mojority of ATF applications is registered thru "trust", instead of an individual. This creates all sorts of issues for LE in case they want to run extra background on you etc. So 90-something % of people chose $50 ATF "trust" that Sheriff cannot technically decline over the other option which is petition as individual and getting close to 100% chance of denial. The biggest hurdle is not some LE not liking your tattoo, its that ATF has a year long backlog. So you pay for your FA $25,000 today but its only delivered to your local licensed gun store, and they won't let you pick it up until ATF sends them document with green light. That take sup to 1.5 year these days. You can come and touch it and take few photos but they will not allow you to load it of course, or to take it out (including shooting range).
Finally, as OT mentions fixed supply, a so-called lower receiver of a fully automatic gun, a piece of metal not much bigger than a door handle (which does not even have any fancy engineering) is literally the most expensive piece of metal that you can buy on American soil. Some examples in extremely good shape go for $35,000 these days (check gunbroker dot com)
In 1986, they stopped accepting new registrations for automatic guns. So there's a fixed supply of legal automatic firearms that normal people can own.
Read more here: https://en.m.wikipedia.org/wiki/National_Firearms_Act#:~:tex....