A semiautomatic rifle is not an assault rifle.
A semiautomatic rifle can only shoot one round per pull of the trigger.
An assault rifle is a military issued rifle that is able to shoot multiple rounds with one pull of the trigger, or has a switch to shoot a burst of two rounds per pull of the trigger, and/or to shoot one round per pull of the trigger. These rifles are already banned in many states, and if allowed, heavily regulated by federal law. A special expensive tax would have to be paid. A federal permit would have to be acquired. A very extensive and personal background check of the person is stored, thus registering the exact identity and whereabouts of all such firearm holders. To acquire this permit, much money and time is required by the applicant. All these assault rifle cost in the range of $10,000-50,000, hence very, very few people own them. And none have been used in a crime for dozens of years. You'll probably never meet a person who owns an assault rifle.
The AR-15 is a semiautomatic rifle. The US military has never used an AR-15 for war.
The AR-15 rifle is a civilian sports rifle. It is used for competitive target shooting, home defense, and hunting, among other lawful things.
Other firearm brands have similar models to the AR-15 platform. AR stands for the initials of one firearm brand, the Armalite company, hence Armalite Rifle.
This platform is highly popular among civilians because of the ability to modify and adjust the cosmetics of the rifle to fit the users personal or special needs/preferences, e.g. shorter/longer buttstock, wider/bigger handgrip, scope or iron sights, color, barrel guard, muzzle flash(fire) suppressor, etc.
This platform is also very light in weight, and is perfect for all sizes of people; often times preferred by women or smaller persons, due to its ability to be adjusted or fitted to any person's size.
Do you realize how ridiculous you sound when you talk about an object you have zero knowledge about?