"The best way to address the firepower concern is therefore not to try to outlaw or license many millions of older and perfectly legitimate firearms (which would be a licensing effort of staggering proportions) but to prohibit the possession of high capacity magazines. By a simple, complete and unequivocal ban on large capacity magazines, all the difficulty of defining 'assault rifle' and 'semi-automatic rifles' is eliminated. The large capacity magazine itself, separate or attached to the firearm, becomes the prohibited item. A single amendment to Federal firearms laws could effectively implement these objectives."
January 1, 1970
https://en.wikiquote.org/wiki/High-capacity_magazine