Service of the piece : 155-MM gun M2 : truck-drawn and tractor-drawn - USA War Department Field Manual, FM 6-90 Service of the piece : 75-mm gun M 2 A 3, truck-drawn and horse-drawn - USA Field Artillery Field Manual, FM 6-56 Service of the piece : 155-mm gun M 1 - USA Field Artillery Field Manual, FM 6-90