The XM307 was born from the since-abandoned "Advanced Crew Served Weapon" (ACSW) project as part of the "Small Arms Master Plan" (SAMP) program of the United States Tank-Automotive and Armaments Command (TACOM). The endeavor intended to produce a modular single-weapon solution capable of replacing both the venerable Browning M2 0.50 caliber heavy machine guns as well as the Saco Mk 19 40mm automatic grenade launchers with one design. The XM307 was designed as a man-portable system that could be fielded by both squad-level elements or be fitted to land vehicles, waterborne vessels or gunship aircraft with little modification. Despite its promising nature, the product was given up for good when it was cancelled in 2007.
The original XM307 initiative fell to General Dynamics Armament and Technical Products and involved complete system revolving around a 25mm grenade launching device that could be converted, within minutes, over to a 0.50 caliber heavy machine gun system (to become the equally ill-fated "XM312" product, detailed elsewhere on this site) as required. This allowed for the gunnery crew to assess the battlefield need and modify their weapon accordingly, fielding the XM307 in an offensive or defensive role as required. As completed, the XM307 weighed in at 80lbs with an overall length reaching 52 inches.
The grenade-launching version of the weapon was chambered for the 25x39mm impact or time-delayed grenade in HE (High-Explosive) or HEAT (High-Explosive, Anti-Tank) flavors. A specialized AP (Armor-Piercing) projectile was also available as the "TP". Training rounds would have been assigned as "TP-S" and been coated in the traditional blue. The impact form allowed the crew to combat light armored vehicles, structures and enemy personnel while the time-delayed feature allowed for air-burst detonation, useful when clearing out enemy elements under cover. The XM307 operated from a gas system with rotating bolt action similar to modern assault rifles of today. The ammunition fed from a 31-round detachable drum affixed to the side of the receiver in a conventional fashion.
Externally, the XM307 mimicked a heavy-caliber machine gun. The receiver was shrouded in a futuristic-looking hard body that encompassed the internal working components as well as the forend and base of the barrel. A carrying handle was affixed over the receiver and set to the right side which facilitated transport. Grip handles (complete with triggers) were set to the aft portion of the receiver in a traditional way. The barrel protruded a short distance away from the body shroud and was capped by a large perforated muzzle brake. The optics system was mounted at the rear of the receiver over the grip handles though this was removed for machine gun function. The entire gun element was positioned on an adjustable tripod featuring two rearward legs and a single forward leg. A typical operating crew was two personnel - the gunner and the ammunition handler.
One of the key engineering qualities of the system was its retarded recoil system which allowed the weapon to be emplaced solely on the provided tripod as opposed to being fixed onto a heavy-duty permanent mounting. The launcher was sighted through a digital day/night viewfinder for accurized fire at range. Rate-of-fire was listed at 250 rounds per minute with an effective range out to 2,180 yards though testing of the available prototypes soon revealed a lower rate unacceptable to US Army authorities. The weapon could reach broader target areas out to nearly 4,000 yards depending on conditions. In this way, the XM307 could be used for both direct and indirect fire against enemy positions as needed.
A remote-controlled version of the weapon, the "Remotely Operated Variant" ( "ROV"), was also under consideration during the XM307's development. This variant would be developed for remote-firing from the safe confines of an armored vehicle, principally the proposed new family of American military armored vehicles known under the collective program name of "Future Combat System". However, this initiative fell to naught as well with the cancellation of the XM307 product and the cancellation of the FCS program itself in 2009.