The vehicle carried 80 37 mm rounds when fitted with a single radio. Vehicles with a second radio installed carried as few as 16 main gun rounds, although unit-level modifications could raise this as high as 40 or more. Machinegun ammunition consisted of 1500 .30 cal rounds and 400 .50 cal rounds. In addition, it carried 16 hand grenades, 4 smoke pots (M1 or M2), 6 landmines (Anti-tank and HE types) and M1 Carbines for the crew.
The armor ranged from 3 mm on the hull floor, to 19 mm on the front hull and turret. The M8 was powered by a Hercules Model JXD in-line 6-cylinder 320 cu.in. gasoline engine giving it a top speed of 56 mph on-road, 30 mph off-road. With a 59 gallon tank, and an average fuel consumption of 7.5 mpg it could manage an average range of 400 miles.
The M8 first saw action in Italy in 1943 and was used by the US Army both in Europe and in the Far East. In the latter theater it was occasionally employed in its original tank destroyer role as most of the Japanese armor was vulnerable to its 37 mm gun. Over 1,000 were supplied via lend-lease channels to Britain, France and Brazil. The vehicle was considered fast, sufficiently reliable (after some technical problems were solved) and armed and armored well enough for reconnaissance missions. However, cavalry units criticized its off-road performance, which was even worse than the M3A1 Scout Car it replaced. In the mountainous terrain of Italy and in the deep mud and snow of North European winter the M8 was more or less restricted to roads, which greatly reduced its value as a reconnaissance vehicle. It was also very vulnerable to landmines. An add-on armor kit was designed to provide an extra quarter-inch of belly armor to reduce landmine vulnerability. Some crews placed sandbags on the floor to make up for the thin belly armour. Another problem was that commanders often used their reconnaissance squadrons for fire support missions, for which the thinly-armored M8 was ill-suited. When it encountered German armored reconnaisance units, the M8 could easily penetrate their armor with its 37mm gun. Conversely, its own thin armor was vulnerable to the 20mm autocannons that German scout cars were equipped with.
The US Army started to look for a replacement for the M8 as early as 1943. Two prototypes, the Studebaker T27 and Chevrolet T28 were finished in summer 1944. Both were found superior to the M8, but it was decided that at this stage of the war there was no more need for a new armored car.
After the war, the M8 was used for occupation duty, saw combat in the Korean War and was retired by the US Army shortly thereafter. France continued to use the M8 until the First Indochina War. Many vehicles formerly used by the US, Britain and France were exported to NATO allies and third world countries. As of 2002, some still remain in service in Africa and South America.
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