The M10’s main armament was a 3-inch gun. The 3-inch rounds had already largely bounced off the frontal armor of the German tanks during the brief engagement before the M10s were disabled. The main gun was not, in the specific circumstances of the disabled vehicle lying in a ditch, available to Murphy anyway — the gun’s ability to bear on targets required a functional turret traverse and a vehicle that could orient itself properly. A tank destroyer mired in a drainage ditch with its main gun unable to bear was not going to be used as a tank destroyer.
The .50-caliber M2 heavy machine gun was a different instrument. It was mounted on the rear of the open turret, and it could be operated by a single person. Its effective range against personnel targets was substantial. It was designed, among other uses, to suppress infantry. Against the 250 German infantry advancing through the fields around Holtzwihr, it was the most effective weapon available.
Murphy was not attempting to stop the German tanks with the machine gun. He was targeting the infantry that the tanks depended on for their combined-arms effectiveness. The accounts describe him hitting a squad that was trying to work through a ditch to flank his position — exactly the kind of infantry movement that accompanies armor in combined-arms tactics. By disrupting the infantry, he disrupted the entire advance.
The artillery he had been directing throughout the action added to this effect. Fire that Murphy had called in through the field telephone — and before the phone line went dead, through continued coordination with the battalion — was landing on the German formation. The machine gun and the artillery together created conditions where the German infantry could not advance and therefore where the German armor lost its combined-arms support.