Most places you need a relatively trouble-free legal history, a driver's license that indicates 21 birthdays behind you, and enough $$ to pay for the gun.
That's not the case everywhere, but a generality that applies to probably 80% of U.S. citizens looking to buy a gun state-side. A few states like to make things more difficult than that.
SI VIS PACEM, PARA BELLUM
People often ask me, "How many ARs do you have?". Well, that seems to be in a constant state of flux. An honest and concise answer might be something like "six and five-eighths". And to the AR enthusiasts among us, that will make perfect sense.