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The M829 is an American armor-piercing, fin-stabilized, discarding sabot (APFSDS) tank round. Modeling was designed at the Ballistic Research Laboratory (BRL) at Aberdeen Proving Ground (APG), which was incorporated into the Army Research Laboratory (ARL) in 1992. The round is specifically modeled for the 120mm M256 main gun on the Abrams M1A1 and M1A2 main battle tanks. The penetrator is carried by a sabot during its acceleration in the gun barrel.
The M829 has a ballistic nose and five tail fins made of aluminum. It is carried in the gun tube by a four-piece aluminum sabot, which separates into four "petals" soon after the round leaves the gun tube. The propulsion system uses an obturating case base with a semi-combustible cartridge wall. It has a total weight of 41.1 lb (18.6 kg) and utilizes a 627 mm (24.7 in) DU penetrator with a 27 mm (1.1 in) rod diameter, which will reach a muzzle velocity of 1,670 meters per second (5,500 ft/s) using of 8.1 kg (18 lb) of JA-2 propellant. Maximum effective range is 3,000 m (3,300 yd). According to Jane's, the M829 is capable of penetrating 540 mm (21 in) of RHA steel armor at up to a 2,000 m (2,200 yd) range. The original M829 is no longer in production and has been succeeded by the M829A1, M829A2, and M829A3. The corresponding training round is the M865 costing $1,121.
The M829A1 (nicknamed the "Silver Bullet" by Operation Desert Storm tank crews) proved itself in 1991 against Iraqi T-55 and T-72 tanks during Operation Desert Storm. The M829A1 round weighs 20.9 kg (46 lb) and has an overall length of 984 mm (38.7 in). The 7.9 kg (17 lb) of JA-19 propellant creates a chamber pressure of 5,600 bars (~81,220 psi), which results in a muzzle velocity of 1,575 m/s (5,170 ft/s). The 684 mm (26.9 in)-long penetrator together with its sabot weighs 9 kg (20 lb). The mass of the penetrator alone is 4.6 kg (10 lb). The effective target range is 3,000 m (3,300 yd)..
The next generation ammunition, called 120mm APFSDS-T M829A2, entered service in 1994, and is the current armor penetrator ammunition being produced by the General Dynamics Ordnance and Tactical Systems for the 120mm M256 gun of tanks M1A1 and M1A2. It is a technology improvement over the M829A1. The new ammunition's performance gains, while classified, result from several new features. These include the use of a special manufacturing process to improve the structural quality of the depleted uranium penetrator. This, plus the use of new composites for the sabot, which, together with a new propellant, provide superior penetrator performance. Combined, these features increase the muzzle velocity of the M829A2 approximately 100 m/s greater than the M829A1 (up to approximately 1,675 m/s), while operating at slightly lower pressure. Projectile length: 780 mm; weight: 9 kg.
On 6 May 2014, the US Army announced that it awarded a US$12 million-contract to defense contractor General Dynamics for the demilitarization and disposal of 78,000 aging depleted uranium (DU) tank rounds as newer rounds are added to the U.S. war reserves. The contract includes M829A1 and M829A2 rounds.
The M829A2 was the immediate response, developed in part to take on this new ERA type. The M829A3 is a further improvement, designed to defeat any future armor protection methods like 'Kaktus' ERA, which was seen on the now cancelled prototype tank, the T-80UM2 "Chiorny Oriol" (Black Eagle) tank. It completed type classification standard in March 2003. Very little is publicly known about the round, perhaps due to export restrictions (see International Traffic in Arms Regulations (ITAR). The M829A3 uses a more efficient propellant, RPD-380, boosting its muzzle velocity. The M829A3 round has a total mass of 22.3 kg (49 lb) and length of 892 mm (35.1 in). It uses 8.1 kg (18 lb) of RPD-380 stick propellant, accelerating a 10 kg (22 lb) depleted uranium rod penetrator, which can be estimated from cutaway mock-ups to be 800 mm (31 in) long, to a muzzle velocity of 1,555 m/s (5,100 ft/s).  From patents submitted by Orbital ATK, the penetrator is comprised of two sections; an approximately 100 mm long steel tip and the rest composed of depleted uranium. The penetrator diameter was also increased from 22 mm (0.87 in) to 25 mm (0.98 in), improving penetrator strength by 67%. This suggests the steel tip is used to defeat the ERA, with the depleted uranium using to complete the penetration through the vehicle's passive armour.  Resulting muzzle energy is 12.1 MJ (8.9M foot pounds). The sabot is of composite material. This variant is unofficially referred to by Abrams tank crews as the "super sabot". It costs around $8,500. Although the M829A3 fired from the 44-caliber M256 gun has a lower muzzle velocity than 120 mm shells fired from the Rheinmetall 55-caliber gun barrel or Russian 2A46 125 mm gun ammunition, it uses a larger penetrator with increased mass to increase imparted kinetic energy.
The A4 (formerly E4) variant was under development by General Dynamics Ordnance & Tactical Systems and Alliant Techsystems (ATK) until ATK received a $77-million, three-year contract on 11 July 2011 to develop and qualify the M829A4 Advanced Kinetic Energy (AKE) round for the US Army's M1A2 SEP (System Enhancement Package) Abrams MBT.
The M829A4 is a fifth-generation APFSDS-T cartridge consisting of depleted-uranium penetrator with a three-petal composite sabot; the penetrator includes a low-drag fin with a tracer, and a windshield and tip assembly. Its propellant maintains consistent muzzle velocities across operational temperatures from −25 to 145 °F (−32 to 63 °C). The new Advanced Combustible Cartridge Case is similar to previous models but has a relocated skive joint placement for better crew-member safety during handling. The initial order for 2,501 M829A4 rounds in 2014 had a unit cost of $10,100 each. On 20 July 2015, Orbital ATK announced that the M829E4 had passed First Article Acceptance Testing and was entering production. On 12 October 2015, Orbital ATK announced the round had recently received type classification as the M829A4, and was awarded a full-rate production contract to begin in early 2016.
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- China Builds the World's Fastest Tank Gun, Then Tries to Hide It - Popsci.com, 19 June 2015
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- ATK To Develop M829A4 120mm Advanced Kinetic Energy Round for M1A2 SEP Tank - Deagel.com, 11 July 2011
- Fifth generation Army tank cartridge reports loudly for duty - Army.mil, 18 April 2014
- Orbital ATK's M892E4 Advanced Kinetic Energy 120mm tank cartridge enters into production - Armyrecognition.com, 22 July 2015
- Orbital ATK has received $105 million domestic and international contracts for ammunition - Armyrecognition.com, 13 October 2015
- M829A1 Technical details and images, from Global Security
- M829 120mm, APFSDS-T, from FAS.org