75 mm Gun M2/M3/M6

The US 75 mm gun tank gun M2 and the later M3 were the standard American tank guns of the Second World War.

Besides use on the two main American medium tanks of the war the M3 Lee (M2 or M3 gun) and the M4 Sherman (M3 gun). The lightweight M6 and M5 variants were developed to equip the Light Tank M24 and the B-25 Mitchell bomber.

Development
U.S. armored doctrine in World War II saw the tank as a deep-attack or exploitation vehicle. The tank's job was to pour through a breach in the enemy front line created by infantry and artillery and exploit that breach by attacking the enemy rear. The tank's primary armament was seen as its machine guns and sheer bulk and crushing power. The main gun was seen as a means of overcoming obstacles as the tank proceeded to attack vital enemy rear areas. For this role the tank gun required good general-purpose performance but anti-tank capability was not paramount. The tank was not supposed to engage enemy tanks. If enemy tanks were encountered in numbers, specialist Tank Destroyer units were to be called in.

History
The 75mm tank gun has its origins in the famous French Canon de 75 modèle 1897 field gun of World War I fame which was also adopted by the United States and used well into World War II as the 75mm M1897 field gun. Both the tank and field guns fired the same range of 75x350R ammunition. The primary round was the M48 High Explosive. This 6.76 kg (14.9 lb) round travelled at 625 m/s (2,050 ft/s) and contained 1.5 pounds of TNT filling and choice of Super Quick (SQ) or Delay (PD) with 0.05 or 0.15 seconds of delay fuse. SQ was the standard setting with PD used against structures, gun positions or lightly protected vehicles. The field gun origins of the ordnance and ammunition ensured that the M2/3/6 series HE round was highly effective for its caliber. The M48 was available in 2 versions, standard or supercharge which increased the propellent charge for greater muzzle velocity (1885 ft/s vs. 1470 ft/s) and range (2,300 yards greater).

Other important rounds fired by the 75mm tank guns were the T30 Canister shot for use against troops in the open at short range. This was essentially a giant shotgun shell full of large numbers of steel balls. Canister was used primarily in the Pacific. There was also the M88 base-ejecting smoke round and the M89 White Phosphorus (WP or "Willy Pete") round which proved highly effective in the Bocage fighting around Normandy. Finally there was the Armor-piercing for which 2 different rounds were provided.

The first armor-piercing round was the 18 lb M72 AP-T, a plain uncapped AP round whose performance dropped off as range increased due to poor aerodynamics. M72 was replaced by the 6.8 kg (15 lb) M61 and later the improved M61A1 APC Shell. This was actually a APCBCHE-T or Armor Piercing, Capped, Ballistic-Capped, High Explosive Tracer type round which contained a tracer element so the gunner could follow the trajectory of the round to make corrections for the next shot if necessary. The aerodynamic ballistic cap acted as a windscreen and improved ballistic performance, maintained velocity, and retained penetration at longer ranges. The armor piercing cap,made of a softer metal, helped to prevent shell shatter at higher velocities and against sloped and "face-hardened" armor. Once the projectile had penetrated the target a small explosive charge at the base of the shell would detonate shattering the shell, and increasing damage inside the enemy vehicle. In practice the majority of M61 rounds were shipped without the explosive filler. M61 had a muzzle velocity of 620 m/s (2,030 ft/s) and was credited with the ability to penetrate 3.7 in of armor plate at 500 yards range, which was a quite acceptable performance by the standards of 1942.

By July 1944 complaints started to pour in about the inadequate anti-tank performance of the M4 Medium tanks fitted with the 75mm M3 gun. During the breakout from Normandy American and British forces encountered the new generation of heavy German tanks and armored vehicles such as the Panther tank, Tiger I tank and Jagdpanzer IV tank destroyer in quantity for the first time. These vehicles had thick frontal armor that proved largely immune to the M61 projectiles fired by the M3 tank gun and severely tarnished the reputation of the M4 Medium tank. The western allies countered by equipping increasing numbers of M4 Medium tanks with the 76 mm gun M1 for the Americans and the Ordnance QF 17 pounder for the British. These guns offered much improved performance against tanks but because they fired High Explosive rounds that were inferior to those of the 75mm guns, the larger calibre guns never completely replaced the older models.

British tanks in the early years of World War II relied on high-velocity anti-tank guns such as the Ordnance QF 2 pounder and Ordnance QF 6 pounder for their primary armament. These guns had the great disadvantage for tank use of not having a truly effective High Explosive round or not even having the option of a HE round. The British after experiencing the effectiveness of the American 75mm tank guns in the infantry support role opted to adopt the American caliber and ammunition by the expedient of boring-out the 6 pounder tank gun to make the Ordnance QF 75 mm. By 1944 this had become the standard British tank gun equipping the Cromwell tank and Churchill tank for the campaigns in northwest Europe.

Variants

 * M2

A version used on the early Medium Tank M3.
 * Barrel length: 31 calibres
 * Muzzle velocity: 588 m/s (1,929 ft/s)
 * Shell weight (M72 AP): 6.32 kg (14 lbs)
 * Armour penetration (M72 AP shell, 457 m, at 90 degrees): 60 mm

Longer derivative of the M2. Equipped American and British vehicles such as the Medium Tank M4, the later models of the Medium Tank M3 and the Churchill III/IV (scavenged from General Sherman tanks in the North African theatre). US Army also experimented with mounting of the M3 on various wheeled carriages for use as anti-tank gun, but the program was cancelled due to lack of requirement.
 * M3

The 75-mm Aircraft gun M4 ia a modification of the M3 gun which is found in medium tanks. It differs from the M3 gun, only in having a seat for the spline machined in the tube. it was mounted on the M6 mount.
 * Barrel length: 40 calibres (3 m)
 * Muzzle velocity: 619 m/s (2,031 ft/s)
 * Shell weight (M72 AP): 6.32 kg (14 lbs)
 * Armour penetration (M72 AP shell, 457 m, at 90 degrees): 76 mm
 * M4

A lightweight version of the M3 with a lighter thin-walled barrel and a different recoil mechanism that was used in the B-25H Mitchell bomber. Uses the same ammunition and has the same ballistics as the M3.
 * T13E1 / M5

A version derived from the T13E1 for the Light Tank M24.
 * M6


 * Barrel length: L/40 calibres (3 m)
 * Muzzle velocity: 619 m/s (2030 ft/s)
 * Shell weight (M72 AP): 6.32 kg (14 lbs)
 * Armour penetration (M72 AP shell, 457 m, at 90 degrees): 76 mm