155 mm gun T7

The 155mm L/40 T7 was an American rifled tank gun developed in 1945. The T7 was to be the main armament for the T30 Heavy Tank, but only a handful were produced due to the T30 project being cancelled after trials in the late 1940s.

155mm L/40 T7
T30 Heavy tank in Detroit Tank Arsenal, July 1950
Type	Tank gun
Place of origin	United States
Service history
Used by	United States
Production history
Produced	1940s
Variants	T7, T7E1
Specifications
Mass	2,384 kg (5,256 lb)
Length	6.64 m (21 ft 9 in)
Barrel length	6.4276 m (21 ft 1.06 in) (L/41.4)
Cartridge	155 × 775mmR
Cartridge weight	M107 HE: 94.99 lb (43.09 kg) projectile, 134.90 lb (61.19 kg) complete M112B1 APBC-HE: 100.00 lb (45.36 kg) projectile, 139.90 lb (63.46 kg) complete T29E1 APCBC-HE: 100.00 lb (45.36 kg) projectile, 139.90 lb (63.46 kg) complete T43 (Mod.) AP:85.21 lb (38.65 kg) projectile, 125.11 lb (56.75 kg) complete T35E1 HVAP:57.18 lb (25.94 kg) projectile, 97.09 lb (44.04 kg) complete
Caliber	155 mm (6.1 in)
Rate of fire	3 rpm
Muzzle velocity	M107 HE: 2,352 ft/s (717 m/s) M112B1 APBC T29E1 APCBC: 2,198 ft/s (670 m/s) T43 (Mod.) AP: 2,533 ft/s (772 m/s) T35E1 HVAP: 3,630 ft/s (1,110 m/s)
Maximum firing range	Project cancelled before maximum range tests occurred

The T7 used two-part separated ammunition like the 105mm T5E1 gun on the T29 Heavy Tank. It had a low velocity of only 701 m/s (2,300 ft/s) compared to the 120mm T53 on the T34 Heavy Tank (945 m/s) and the 105mm T5E1 on the T29 Heavy Tank (945 m/s). However, the 43 kg (95 lbs) High-Explosive shell (HE) and high-velocity armour-piercing (HVAP) was demonstrated to have a powerful demolition effect. Testing concluded before completion when the T30 project was cancelled in the late 1940s.

Ammunition

Previous developments

Main article: 155 mm gun M1

Development

Document Approval of the design of the 155 mm T7 gun September 14, 1944

The 155 mm T7 L/40 (41.4 Caliber) is a 155 mm gun of the T30 heavy tank, developed from a shortened 155 mm M1 L/45 “Long Tom”, conceived as early as 14 September 1944. It was developed alongside with the T29 heavy tank armed with 105 mm T5E1 L/65 gun. The T30 had up to 5 different ammunition available for use. Starting from M107 HE, M110 WP, M112B1 APBC-HE, T29E1 APCBC-HE, and finally T35E1 APCR. The specification will be listed as fired from the T7.

HE M107

155mm M107 HE Shell

A standard high explosive shell used on all American 155 mm gun in WWII. Weighing 95 lb (43.09 kg), with an explosive filler of 15.13 lb (6.86 kg) TNT, this was primarily used as anti-fortification. It was also a default shell of the T30.

• Specification:
• Weight: 94.99 lb (43.09 kg)
• Muzzle velocity: 2,352 ft/s (717 m/s)
• Explosive type: TNT
• Explosive weight: 15.10 lb (6.85 kg)
• Fuze: P.D. M51A4

APBC-HE M112B1

155mm M112B1 APBC-HE Shell

An armor-piercing round. Weighing 100.00 lb (45.36 kg), with an explosive filler of 1.44 lb (0.65 kg)) Explosive D, it was used as an anti-concrete shell, or as an anti-tank in secondary tank destroyer role. The difference between M112 and M112B1 is the cap. Performance-wise, both are identical. V50 penetration table on various AP projectiles tested by the Americans from the Canadian AFV Technical Situation Report No. 34, showing some well-known shell in service during WWII, including the 90 mm T33, 155 mm T7 gun fired 100.00 lb (45.36 kg) AP M112B1 at 2,200 ft/s (670 m/s) can penetrate a homogeneous armor plate, at only 228 mm penetration from point blank against vertical armor

• Specification:
• Weight: 100.00 lb (45.36 kg)
• Muzzle velocity: 2,200 ft/s (670 m/s)
• Explosive type: Explosive D
• Explosive weight: 1.44 lb (0.65 kg)
• Fuze: B.D. M60

APCBC-HE T29E1

Documents T29E1 shells for 155mm T7

The ballistic limit given is 2,533 ft/s (772 m/s) against 203 mm at 30° from vertical, measuring the 30° point blank penetration from given ballistic limit results 200 mm. With APCBC slope multiplier against vertical armor, that would mean about 284 mm from point blank range, substantially higher compared to the M112B1

• Specification:
• Weight: 100.00 lb (45.36 kg)
• Muzzle velocity: 2,533 ft/s (772 m/s)
• Explosive type: Explosive D
• Explosive weight: 1.44 lb (0.65 kg)
• Fuze: B.D. T70

HVAP-T T30

The 155 mm T35 HVAP, and 90 mm M304 for scale

This was the largest APCR projectile to be developed for a tank gun in WWII. Designated as Shot, H.V.A.P., 155MM, T35E1, or just simply called as T35E1 HVAP. Grown out of the concern from encountering enemy heavily armored tanks, the U.S. Army Ground Forces was pressed to develop a necessary anti-tank munition for their latest heavy tank in development, the T29 and T30. These tanks were intended to support allied forces with combined heavy armor and heavy firepower to defeat the most well-fortified enemy positions with different roles. The T29 would be used to combat other tanks with its 105 mm gun, and the T30 would be relegated for anti-fortification with its 155 mm gun. Leaving the T29 aside since it was built to confront hostile armor anyway, the T30was stuck with no self-defense munition to hit back should it come across opponents like King Tiger or Jagdtiger. To quote the Office Memorandum from the U.S. Office of Research and Development about the T30 Heavy Tank project (8 February 1945)

“While the 155mm gun in the T30 is intended to deliver maximum effective HE fire, a new HVAP projectile, T35, having a muzzle velocity of 3,630 ft/s (1,110 m/s), has been designed for this gun. This projectile will penetrate 9″ of homogeneous armor at 30° obliquity at a range of 2000 yards. This tank is a companion vehicle to the hole-punching, heavy Tank T29, and is deemed essential in order to keep pace with enemy tank developments.”

A new anti-tank munition was then developed and designated as the 155 mm HVAP T35. Apart from looking comically large for a subcaliber shot, structurally, its design was similar to the 90 mm M304 HVAP with multi-piece carrier construction, consisting of aluminium body fitted with a steel bourrelet ring, an aluminium windshield, and a steel base fitted with a copper driving band and trace. However, the most ridiculous of all was the amount of penetrating core inside it. a whopping 14.9 lb (6.8 kg)) worth of tungsten carbide was crammed inside the projectile, with an estimated core diameter of 60 mm. This had the same weight of a single full-bore 76 mm M79 AP shot, but instead of monobloc steel, it was much harder tungsten carbide. The total weight of the entire projectile including the penetrator was 57.18 lb (25.94 kg). Test firing would be done using the T7 gun firing at full charge with a muzzle velocity of 3,630 ft/s (1,110 m/s). Its penetration was quite high, 392 mm versus RHA at 0° from point-blank. Compared with the 105 mm T29E3 HVAP of the T29, capable of penetrating up to 379 mm of RHA from similar setup.

Compare armor penetration values

Estimated penetration table for the new anti-tank projectiles.

Documents reporting values from T7 and T5E1 guns

In conclusion, the 155 mm T7 was a medium velocity demolition gun mainly purposed for bunker busting, But in a scenario where it was forced to engage heavily armored vehicles, the T35 Projectile hyper-velocity shot was created to provide an effective countermeasure. It probably costed a fortune for the Ordnance Department to develop though, and was highly inefficient production-wise by wasting that much tungsten carbide for a single 155 mm HVAP, which could’ve been used to create three more 76 mm HVAP instead). The T35E1 HVAP project was terminated along with the T30 Heavy Tank postwar, partly due to the manufacturing cost, and the end of hostilities in both Europe and Pacific.

• Specification:
• Weight: 57.18 lb (25.94 kg)
• Muzzle velocity: 3,630 ft/s (1,110 m/s)
• Core diameter: 60 mm (est.)
• Core mass: 14.9 lb (6.8 kg)
• Core type: Tungsten carbide

M110 WP (Smoke)

155mm M110 Smoke For the T30 heavy tank

Smoke shell, used to provide smoke screen with 15.58 lb (7.07 kg) of white phosphorous. The ballistics and configuration are similar as the M107 HE.

• Specification:
• Weight: 98.39 lb (44.63 kg)
• Muzzle velocity: 2,352 ft/s (717 m/s)
• Smoke type: WP
• Smoke weight: 15.58 lb (7.07 kg)
• Fuze: P.D. M51A4

Penetration comparison

90mm Shell (left), 155mm Brass Shell Casing (center), 155mm T43 (Mod.) AP Projectile (right) and 20mm shell (bottom corner)

Calculated penetration (0-30 degrees)

Ammunition type	Muzzle velocity	Penetration (mm)
		500 m	1000 m	1500 m	2000 m	2500 m	3000 m
M112B1 (APBC-HE)	670 m/s (2,200 ft/s)	215	213	203	211	195	187
T43 (Mod.) (AP)	770 m/s (2,500 ft/s)	254	243	236	226	215	203
T35E1 (HVAP)	1,106 m/s (3,630 ft/s)	392	355	340	314	292	276
M107 (HE)	701 m/s (2,300 ft/s)	85	85	85	85	85	85

Variants

• T7 - Standard model
• T7E1 - T7 modified for use with a power rammer and ejection equipment.

Bibliography

• Hunnicutt, Richard Pearce (1988). Firepower: A History of the American Heavy Tank. Novato, California: Presidio Press. ISBN 0-89141-304-9.