The full auto weapons you refer to are ones manufactured or imported before 1986; that’s why, even if legally qualified, you can’t buy a full auto for much less than $15,000. All those either imported or manufactured after 1986 are permanently off limits to the civilian market. Short supply plus high demand equals high prices.
I’m aware of all this however the point is if you want a full auto you can get one.
Personally I prefer a semi auto as it requires you to be a rifleman. The key is one shot one kill. Spray and pray is for the bros in the Hood.