Armored personnel carrier T113
LOW GROUND PRESSURE VEHICLES
In February 1965, the U.S. Army initiated the development of the XM733 assault vehicle as part of the Remote Area Mobility Study (RAMS). This was the same study that resulted in the XM729. The XM733 was essentially an armored M116. It was an open top vehicle armed with 7.62mm and .50 caliber machine guns. A 40mm grenade launcher also could be installed. On the XM733E1, a cupola was provided to protect the gunner.
The Army canceled the XM733 program in October 1966, but the Pacific Car and Foundry Company received a contract to manufacture 93 of the light armored vehicles for the U.S. Marine Corps. On 22 December 1966, Pacific Car and Foundry was awarded another contract to build 111 unarmored XM733s for the U.S. Navy and Marine Corps. Since an unarmored XM733 was essentially an M116, these vehicles were designated as the M116A1 to avoid confusion. The first production M116A1 was delivered to Aberdeen Proving Ground in December 1967.
The M116 also was proposed as a carrier for the M29 Davy Crockett Battle Group Atomic Weapon System.
FIRE SUPPORT VEHICLES
As mentioned earlier, construction was authorized in May 1955 for two experimental self-propelled mortars along with the original T113 pilots. In October 956. the preliminary design was reviewed and approved for development by FMC. The new vehicle, based upon the armored personnel carrier T113, was lesignated as the 81mm, full-tracked, self-propelled mortar T257 in August 1957.
At that time, further development of the T82 self-propelled mortar was canceled. The T257 pilots constructed at FMC were delivered in March 1958. These self-propelled mortars used the basic chassis of the T113 with aluminum alloy armor and the trailing idler suspension. Like the T113, they were powered by the AOSI-314-2 air-cooled engine with the X-drive transmission. The vehicles were armed with the 81mm mortar M29 on an M23A1 mount in the troop compartment.
A circular folding roof hatch opened to provide a clear field of fire for the mortar. A beam across the bottom of the troop compartment supported the mortar mount to absorb the shock loads from firing. At first this was a steel beam, but it was replaced later by an aluminum beam welded into the hull structure. A circular base for the rotator was welded on top of the beam.
The rotator itself was secured by a retaining ring bolted to the circular base. This ring had 56 teeth on the inside surface to lock the rotator in any one of 56 positions in the 360 degrees of traverse. The traversing mechanism in the mortar mount permitted further adjustment of the mortar in traverse.
The feet of the mortar mount bipod were secured by locking pins in a supporting bracket. This bracket held the mortar in firing position or the weapon could be lowered to a stowed or travel position. A separate base plate was carried so that the mortar could be removed from the vehicle and fired from the ground.
Tests during 1958 revealed some slight deformation in the rotator base ring after firing and modifications were recommended. After evaluation by CONARC, the T257 was considered suitable for troop use. However, the armored personnel carrier T113 upon which it was based had been superseded by the modified T113E2 now standardized as the M113 armored personnel carrier.
A new directive also required that the 81mm and 4.2 inch mortars be interchangeable in the new carrier. FMC constructed a mock-up of the modified vehicle now designated as the 81mm/4.2 inch self-propelled mortar T257E1. Based upon the M113 armored personnel carrier, it was powered by the same Chrysler 75M engine with the Allison TX200-2 transmission.
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