Semi Automatic Rifles have been around for many years. The AR style platform you see depicted here is NOT an Assault rifle. The politicians and anti-gun lobbiest believe AR stand for Asault Rifle when in fact it stand for Armalite Rifle. Armalite is the company to first make this style of semi automatic rifle.
While the AR platform is the most notible semi automatic rifle there are many other styles used in hunting, home defense and target practice. You will find calibers as small as the 17 to as large as a Barret 50 cal. They all have their place in the market.
If you are qualified and approved with a Federal Background check you should be able to purchase these.