Draft:XM274 (Tank Gun)

The Ares Incorporated XM274 (originally designated as the medium calibre, antiarmor automatic cannon, MC-AAAC) was an American smoothbore 75 mm autocannon. It used a rotating breech block with cased telescopic ammunition. It was developed for the HIMAG program, but was later used on a variety of test vehicles.

History
In the 1970's development of new types of ammunition such as ATGM's and HEAT with shaped charges and long rod kinetic penetrators made it seem that heavily armoured vehicles such as MBT's were obsolete. However there are other forms of survivability for armoured vehicles, such as mobility, which has always been the primary factor in the survivability of light vehicles.

In 1972 DARPA initiated a study to see the effect of new technologies such as new weapons systems would improve the effectiveness of combat vehicles. Under this program, in 1975 a proposal for a high performance test bed was made, it was to evaluate, among other things, a high velocity, high fire rate medium caliber autocannon. The vehicle would begin to be built in 1976 and designated the High Mobility Agility (HIMAG) vehicle. A 75 mm smoothbore CTA autocannon would be made for the vehicle, originally designated as the medium calibre, antiarmor automatic cannon (MC-AAAC), later designated XM274. The gun was to be developed under a DARPA contract by Ares Incorporated. The original contract specified a 60 mm, but it was later changed to be 75 mm. The original design for the gun used a sliding breech, however after a failure to meet the required dispersion during testing in January 1979, the gun was instead switched to use a rotating breech. The complete HIMAG system, and thus the MC-AAAC, were sent to the Abeerdeen Proving Ground in 1980, for evaluation by the US army.

The HIMAG was then shipped to Fort Knox were it completed tests in 1981, and was then later used for testing in other programs. The MC-AAAC was found to have similar dispersion to a large calibre tank gun (105 /), and 2 round bursts were found to be the most effective method of shooting on the move. In 1976, the HIMAG project was moved from being a DARPA contract to being under the newly formed Armoured Combat Vehicle Technology (ACVT) program, jointly run by DARPA, TACOM and the USMC. Under this new program, another vehicle using the MC-AAAC was developed, the High Survivability Test Vehicle - Lightweight (HSTVL). The HSTVL had it's origins in early proposals for the HIMAG. From February to July of 1977 both AAI Corporation and Pacific Car and Foundry Company each designed and developed a proposal for the HSTVL project. They were both armed with the MC-AAAC, with the AAI proposal mounting it in a conventional turret with a 60 round magazine and the PCAF proposal mounting the gun in a external elevating trunnion with 22 rounds ready.

After evaluation of both proposals in late 1977, the contract was awarded to AAI, who then built the vehicle and it was then used in testing as it was never intended to be a production vehicle. While PCAF lost the contract for the HSTVL, they went on to develop the elevating trunnion gun mount further. PCAF received a contract to build the Elevated Kinetic Energy (ELKE) vehicle which was completed and sent off for testing in 1982. It mounted the MC-AAAC in a external elevating trunnion mount with a very wide range of elevation and depression angles, with the turret mounted on the hull of an M551.

Also under the ACVT program, 25 different vehicle design concepts were produced, including scout vehicles, IFV's and light tanks. Most of these were to be equipped with the 75 mm Ares gun, however a few were to be equipped with a scaled up 90 mm version of the gun, of which little information is known about, only that it was built and tested but never used in a vehicle. None of these design concepts were ever built.

In 1980, AAI further developed the HSTVL concept into the Rapid Deployment Force Light Tank (RDF/LT) prototype. This was, like the HSTVL, to be armed with the MC-AAAC which had now officially been designated as the XM274. As the RDF/LT was intended to have a secondary anti air role, a 75 mm proximity fused HE round was made for the gun. As the XM274 was not permitted for export, another version of the RDF/LT was proposed armed with the M32 76 mm used on the M41 for which a new APFSDS round was developed. A towed variant of the gun was also proposed, to be carried on a M31 gun carriage used by the M102 105 mm howitzer, with a video system mounted on the gun so that it could be remotely aimed and fired with the gun feeding from a 6 round magazine. By the early 1980's, it was becoming apparent that the 75 mm would no longer be suitable for use against more modern armor and so all new proposals for light vehicles intended to fight tanks were to instead use a larger caliber gun such as the 105 mm M68 and as such no further vehicles were produced with the XM274, though any test vehicles using the gun may have still be in use at this time.

Design
The main component of the gun is the rotating breech block. This worked by allowing the breech block to rotate to the vertical so that a round could be loaded from an autoloader, then the breech block would rotate back to the firing position to be horizontal with the barrel. When the gun was fired, the casing would then expand to seal the gap between the casing and the barrel. After firing the breech block would then rotate again to the vertical, the spent case would then be ejected from the top of the breech block by the next round being rammed up into it. A self contained hydro-pneumatic recoil system was used. The maximum rate of fire was 2 rounds a second, however the FCS and stabilization was not able to keep up with this and so operationally the rate of fire was set to 1 round a second. For all applications an autoloader was used with the XM274, however the type of autoloader varied depending on which vehicle it was mounted in, in the HIMAG it used a 6 round vertical carousel and in the HSTVL a 60 round vertical magazine was used. The gun was also designed to be lightweight, using a composite gun sleeve which also had holes cut into it to reduce weight, the recoil mechanism was also incorporated into the breech reducing it's weight further.

Ammunition
All the types of ammunition used were cased telescopic. The projectile was embedded in the solid propellant (liquid propellant was originally intended, but later moved to a separate project), shortening the length of the round and allowing a rotating breech block to be used and increasing the rate of fire. All types of ammunition were 406 mm in length and 132 mm in diameter, with each weighing 15.6 kg.


 * XM884 HE that could use a standard fuse or a proximity fuse for use against aircraft.
 * XM885 APFSDS-T - there are 2 known variants, both of which had DU penetrators . Delta-3, which used a 2.27 kg penetrator with equivalent performance to 105 mm M774. Delta-6, which has a unknown penetrator mass with 430 mm of penetration into a 450 mm steel target.
 * XM886 Target Practice TPFSDS-T
 * XM887 Target Practice TP-T

Usage

 * HIMAG
 * HSTVL
 * RDFLT
 * ELKE
 * ACVT Program