The object of this clause is to secure a well-armed militia … But a militia would be useless unless the citizens were enabled to exercise themselves in the use of warlike weapons. To preserve this privilege, and to secure to the people the ability to oppose themselves in military force against the usurpations of government, as well as against enemies from without, that government is forbidden by any law or proceeding to invade or destroy the right to keep and bear arms.

Source: John Norton Pomeroy