Blackstone;
The United States was originally formed of 13 former colonies which after gaining their independence considered themselves sovereign nation states (which is why they're called states, not provinces as in Canada). The states were not mere administrative subdivisions of the national government any more than the UK is a mere subdivision of the EU.
In fact, the US was set up much like the EU: sovereign states establishing a larger, but limited, government to regulate a few carefully delineated functions such as a common currency, uniform trade policies, mutual recognition of one anothers' citizens, and courts to arbitrate disputes between states and their citizens.
One function of the new US Federal government was creation of a standing army. Bear in mind, the politicians of the day were men who had seen how a standing army could be used to dominate a civillian populace, and were quite determined that each sovereign state should have militia (an unorganized body of armed citizenry to be called out in defense of the state and its citizens). Allowing the federal government to disarm the populace would leave the citizenry of a state at the mercy of a standing army controlled by the federal government.
Now reread the text of the Second Amendment. "A well regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed."
Now argue if you want as to what may constitute a "well regulated militia"; it should be very clear that the nation's founders clearly intended the 2A to be a blanket prohibition against the federal government having any sort of regulatory control over private ownership of weapons. It was intended to be a check and safeguard against attempts to abuse the power of the federal government.
That check and safeguard is still needed. This is why we fight any, each, and every new attempt to give the federal government a power it was not meant to have.
