Because the AR-15 is the civilian version of the M-16. Every American who has served in the military and most who have been in law enforcement during the past 40 years has trained with and carried the M-16. The similar AR-15 currently provides millions of Americans with the most familiar and effective arm for common defense and self-defense.
According to the U.S. Supreme Court, the Second Amendment guarantees the right to bear weapons that are “part of the ordinary military equipment”, that “could contribute to the common defense” and “of the kind in common use” at the time in question. U.S. v. Miller, 307 U.S. 174 (1939) @ 178-9. The M-16 fits this definition without any doubt. The AR-15 is the commonly owned civilian version “typically possessed by law-abiding citizens for lawful purposes”. D.C. v. Heller, 554 U.S. 570 (2008) @ 625.
Therefore, the proposed “ban” on AR-15s will violate the Constitution. The Second Amendment must be repealed or modified to avoid this violation. Further, legislating to restrict ownership of the AR-15 in hopes that the courts will not recognize the historic and legal meaning of the Constitution seems unlikely. The Supreme Court already stated “that it is not the role of this Court to pronounce the Second Amendment extinct.” Heller at 636.