It's a M1857 Light 12pdr, or "12 pounder Napoleon". The smooth, undecorated exterior of the tube (aside from the muzzle flare) is what identifies it.
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“[B]ut these tanks are machines, their caterpillars run on as endless as the war, they are annihilation, they roll without feeling into the craters, and climb up again without stopping..." -Erich Maria Remarque
