If u are afraid of an invasion, organize a home guard like European countries (or use the NG) so u have an organisation with a command structure, supply lines etc. Its very naive to belive that a bunch of civilians with rifles can put up any resistance in a conventional war against an army. Guerilla warfare is a different thing but even then it comes with a high prize. And even then it will take quite some time before the guerillas has gained enough experience to be effective.
But as Kampfer said: In neither case is it neccessary to bring the gun to the store, there will be enough time for u to go home and get the gun before the red horde is upon u.