The Largest APCR Projectile on a Tank Gun: T35E1 HVAP

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
The 155 mm T35E1 HVAP, and 90 mm M304 for scale.
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 was built to confront hostile armor anyway, the T30 was stuck with no self-defense munition to hit back should it come across opponents like King Tiger or Jagdtiger.
Heavy Tank T30, the primary and only user of 155 mm T7 L/40 cannon.
The Heavy Tank T30 mounted the massive 155 mm T7 L/40, a cut down derivative of the 155 mm M1 L/45 “Long Tom” heavy artillery cannon adapted for tank use. Combat records from the frontline had proven the effectiveness of large caliber cannon against anti-personnel, anti-concrete, and counter battery missions compared to smaller caliber one. As such, the project was split between the T29 and T30 primarily in regards to main armament. The T30 was to be considered as “assault gun” to provide close support for friendly infantries or tanks. This way, it could deliver direct heavy artillery fire against pillboxes, bunkers, MG nests, or AT emplacements. The T7 gun would initially use the standard issue M107 HE and M112 AP shells from the M1 gun, but with smaller propellant charge since long-range fire was not necessary for “assault gun” role, and to cut the loaders some slack with loading the shells inside an enclosed turret. Unfortunately, this ammunition load was simply not enough.
M107 HE and M112 AP, the primary loadout for the 155 mm T7.

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 3100 f/s, has been designed for this gun. This projectile will penetrate 7″ 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 15 pounds (6.8 kilograms) 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 25.62 kg. Test firing would be done using the Long Tom gun firing at full charge with a muzzle velocity of 1106 m/s.

Shot, H.V.A.P., 155MM, T35
Weight: 25.62 kg (56.5 lb)
Core mass: 6.8 kg (15 lb)
Core diameter: 60 mm (2.36 in, est.)
Core type: Tungsten carbide
Muzzle velocity: 1106 m/s (3630 ft/s)

Its penetration was quite high, estimated around 392 mm versus RHA at 0° from point-blank. But this shot was used for the 155 mm M1, not for the 155 mm T7. A new shot was designed instead specifically for the latter, designated as the 155 mm HVAP T35E1. As with the T35, a 15 pound tungsten carbide would be retained, but with slight differences to the projectile body, resulting in an increased total weight of 25.94 kg. The muzzle velocity was greatly reduced when fired from the T7 gun, only 945 m/s.

Shot, H.V.A.P., 155MM, T35E1
Weight: 25.94 kg (56.5 lb)
Core mass: 6.8 kg (15 lb)
Core diameter: 60 mm (2.36 in, est.)
Core type: Tungsten carbide

Muzzle velocity: 945 m/s (3630 ft/s)

With significantly reduced muzzle velocity, it only penetrated about 316 mm versus RHA at 0° from point-blank, surprisingly similar to the 90 mm M304 HVAP used by the U.S. late war vehicles such as the M36 Jackson and M26 Pershing. Yet, even with a gigantic subcaliber shot, it was still inferior to the 105 mm T29E3 HVAP of the T29, capable of penetrating up to 379 mm of RHA from similar setup. Fortunately, it didn’t need to outperform it.
Estimated penetration table for the new anti-tank projectiles.
In conclusion, the 155 mm T7 was a medium velocity demolition gun mainly purposed for bunker busting, not to punch a hole through other armor like the 105 mm T5E1. But in a scenario where it was forced to engage heavily armored vehicles, the T35E1 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.

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