Ask the person you're arguing with just what, exactly, is an assault rifle. Odds are they're going to say "a machine gun". Machine guns have been heavily regulated since 1934, and assault rifle (or weapon) is a buzzword made up by politicians to take guns away from people based on physical appearance only.
Furthermore, assault is a behavior, so anything in the world can be an "assault weapon". I could throw my keyboard at someone and, because I used it as a weapon to assault someone, it is now an "assault weapon". I know, it's stupid.
Also, a Bill of Justify Your Necessity Before Exercising These Rights has never been ratified by these United States, ever in our history. The same question could be posed as...
"Why do rock singers always buy Ferraris when a yugo is just as good for getting from point A to point B". Well, a Ferrari is a whole lot more fun than a Yugo, and a CAR-15, with all the bells and whistles that would make a liberal college student literally crap their pants, is a lot more fun than a muzzle loader to shoot (personal opinion).
It's the Bill of Rights. I'll be thrice damned before I justify my right to own an assault rifle to the government, which is limited by the same document. They are the ones that need to justify their actions to US, not the other way around.
Most assault rifles are NOT designed to kill people. They aren't powerful enough (with the notable exception of the AR15). Their purpose is to wound people to take as many enemies out of the fight as possible as they take care of their wounded comrades. A semi-automatic pistol would do more damage with the right rounds than a FMJ assault rifle round.
The Militia Act of 1797 (I think) says that all able bodied males aged 17-45 are members of the unorganized militia, and therefore must own a weapon of military usefullness and be proficient with it's use. If you do not own a weapon of military usefullness, you are breaking the law and are derelict in your duty as a member of the militia.
Just my .02.
