Authored By Dan Alex (Updated: 8/3/2015):
The M7 Need and Its Development
The need for a self-propelled artillery system proved ever apparent since the beginning stages of World War 2. The Germans were quick to acknowledge this need and developed obsolete and even captured systems to fit the bill - usually just mating a powerful field gun or tank-killing weapon to an improvised superstructure atop a proven track-hulled system. Perhaps the only difference between the parties involved was in doctrine - the Germans choosing to field their self-propelled artillery systems in a direct fire role while the Allies generally kept their systems behind the front lines in the indirect fire role.
The US Army itself had already experimented some with the fitting of a 105mm field gun to the existing chassis of their half-tracks but these led to limited successes. What it really needed was a fully-tracked implement utilizing existing components to help facilitate production and keep costs at a minimum while offering unparalleled cross-country performance and fire support for infantry and armor actions during a given operation. The chassis of the M3 Lee Medium Tank was selected as the starting point for it was available in some number and the tracked qualities of her hull offered up the required cross country mobility - a component that would allow the new weapon system to better support the expected armored thrusts across the muddy and hilly European terrain. The upper hull was extensively redrawn and rearranged to make room for the form and function of the M1A2 105mm howitzer.
A pilot vehicle was penciled out under the designation of "T32" and two prototypes were constructed. Self-defense would be handled by a modest addition of a single Browning M2 .50 caliber air-cooled machine gun in a raised cupola. Twenty-four 105mm projectiles would be displaced about the superstructure along the inside of the left and right side walls as well as under the floor, each projectile housed in individual protective cylinders. Armor was 51mm (2-inches) at its thickest point and power was derived from a single Continental R975 C1 series 9-radial engine of 350 horsepower running at 2,400rpm. Suspension was handled a vertical volute spring system. The T32 was placed through an appropriate series of trials at Aberdeen Proving Ground and eventually culminated with the "M7" in February of 1942. Production was slated to begin in April of that year.
M7 Production Begins and Origin of the Priest Name
From April of 1942 to August of 1943, the American Locomotive Company (ALCO) was responsible for construction of 2,814 M7 systems. Via Lend-Lease, 90 of these early examples were shipped to the British 8th Army fighting in Egypt and immediately took on the nickname of "Priest" thanks to the pulpit-like drum-style cupola mounting the .50 caliber machine gun. The British were quick to remove 22 of the 105mm projectiles in favor of installing vital radio communications equipment in their place.
Some revisions were already underway while these early systems were being delivered and this produced three slightly varied forms. Most notable of the revisions was to the ammunition stowage compartments, now increasing hold for up to 69 projectiles of 105mm ammunition.
M7 Priest Walk-Around
Design-wise, the M7 sported a lower hull seemingly identical to the M3 Lee medium tank series. She showcased the same six-wheeled track systems with each mounted to the hull sides in pairs and braced by the suspension system. The drive sprocket was at the front, tied to the transmission system which, itself, was paired to the engine held in a compartment to the rear of the vehicle. The superstructure was a large, fixed formation of slab-sides and seated atop of the hull. The glacis plate was slightly angled and contoured into the forward superstructure facing. The driver was situated to the left of the superstructure and managed the mobility functions of the vehicle. He was afforded a rectangular viewing port that could be left open or closed as actions dictated. The 105mm main gun, and applicable firing system, was situated to his right and offset just right of the vehicle's centerline. As the main gun was seated within the superstructure walls itself, this meant limited side-to-side traverse. To help promote a lower silhouette, the main gun was also originally limited to a 35-degree elevation firing arc. To the right of the main gun mount was the pulpit/observation post usually fitting a single Browning M2 .50 caliber heavy machine gun for anti-infantry/anti-aircraft defense. The superstructure was "open-topped" to help facilitate observation of the battlefield ahead. As such, the crew was unnecessarily exposed to enemy fire, fragmentation effects and the elements. This design element, however, was common practice for such weapon systems during the war. Reported speeds were 24 miles per hour on roads while this degraded to 15 miles per hour off road. Range was approximately 120 miles.