No Marines left at the Marine Barracks? Then just start passing out M-1s at the Pentagon. There should be enough senior officers there to make up a battalion.
M-1s? That’s so 1944 but would still be effective.
Unfortunately the only M-1 Garands left in the Army inventory are in its museums. And none of today’s soldiers have trained on them. All are or should be familiar with M-16s and/or M-4s.