The purpose of the Second Amendment is to arm people in order to prevent future tyranny. They need the tools to do this.
The term "Well Regulated" in the Second Amendment meant "Well Manned and Equipped " in 1791 as was determined in the 1939 United States v. Miller case after referencing the autobiography of Benjamin Franklin. The concept of Government Regulation, as we understand it today, did not exist at the time.
United States v. Miller also determined that the term "Arms" refers to "Ordinary Military Weapons" (not crew operated). American Citizens have the right to Keep and Bear, which means Own and Carry, any weapons that a soldier carries into battle. That includes past, present and future weapons.