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The M1 and M1A1 had a plain barrel without cooling fins, and a simplified rear sight. It also had the cocking handle on the side of the receiver, rather than the top. We prized it for  its firepower, though its hefty weight made it an armful on an extended patrol. Unfortunately it had limited effect in heavy jungle cover, where the low-velocity .45 bullet would not penetrate most small-diameter trees, or Japanese helmets. But what can you say about a gun that would save your life?