During July of 1944, the American 630th Tank Destroyer Battalion accounted for no less than 53 Panther medium and Tiger I heavy tanks in ensuing battles. The group also managed to net no fewer than 15 German self-propelled tracked vehicles during the same span. Of note among these totals was that only 17 Hellcats were lost to enemy action or other factors. It was this sort of kill-to-loss ratio that M18 crews enjoyed through the course of the war. The development of High Velocity Armor Piercing (HVAP) later in the war only served to make their 76.2mm guns more potent and increase penetration values against German targets.
Like the M10 before it, the M18 soldiered on in its tank destroyer battalions until it was rationalized that dedicated tank destroyer groups were no longer needed considering the advances made to combat tanks in general. Future armored warfare would rely strictly on tank-versus-tank battles that did not require the intervention of dedicated tank destroyers per se. As a result, American tank destroyers were fielded more and more as part of the direct armored force and not held in the reserve role, being called to action when needed. This placed such units more in line with the assault and self-propelled artillery roles in which they functioned alongside infantry actions and were further supported by air cover. The Allies enjoyed air superiority towards the end of the war which limited the reach of German assaults, especially on good weather days. The dedicated tank destroyer concept was effectively dead by war's end and dropped from American Army doctrine thereafter.
Such was the success of the M18 hull and powertrain that the US Army was quick to develop other useful battlefield roles for the type. A late-war development saw the fitting of a 150mm field howitzer to the M18 chassis to create the "T88 Howitzer Motor Carriage" prototype. This self-propelled artillery platform was undergoing testing when the war ended and subsequently saw no production beyond the prototype stage. The "90mm Cannon Motor Gun Carriage" attempted to fit a 90mm high-velocity main gun to the M18's turret but the end of the war stopped its progress. The "T41E1" prime mover and "M39" fast infantry carrier were also developed - each sans the turret of the original M18 - though the T41E1 fell to naught with the end of the war and the M39 (more formally as the "Armored Utility Vehicle M39") was produced in approximately 650 examples. Up to eight personnel could be carted in the M39 which was armed with 1 x 12.7mm Browning M2HB heavy machine gun for self-defense and these saw action in the upcoming Korean War. The "T65 Flame Tank" as well as the "T86", "T86E1" and "T87" amphibious howitzer vehicles all failed to make it to production. Other prototypes trialed included a mobile command post.
Design of the M18 was highly conventional and followed common American tracked vehicle practices of the time. The powerpack was settled in a compartment to the rear of the vehicle with the drive sprocket at the front of the hull. The track idler was set to the rear and four track return rollers managed the upper portion of the track systems themselves. There were five rubber-tired road wheels per track side with little in the way of side armor skirting for point defense against side direct hits. The turret was set just ahead of amidships and fitted the long barrel main gun, capped by a double-baffled muzzle brake (this found on M1A2 guns). The sides of the hull superstructure sported clean edges and were sloped inwards for ballistics protection. The open-topped turret was oblong from the overhead profile with an overhang at the rear for maximum internal stowage. The turret also featured sloped sides and an armored front facing for maximum protection within the required design weight limit. A close-in heavy machine gun was fitted to a flexible ring mount at the rear of the turret and could be operated by any of the turret crew.
The M18 was powered by a Continental R-975 C 9-cylinder radial, air-cooled, gasoline-fueled engine delivering 340 to 400 horsepower. This supplied the vehicle with a top speed of 55 miles per hour and an operational range of 150 miles. She held a crew of five including a driver, co-driver, a dedicated gunner, an ammunition handler and the tank commander. Primary armament was the 76mm (3 inch) M1A1 or M1A2 main gun with secondary armament - mostly for defensive purposes - supplied by a 12.7mm Browning heavy machine gun. The main gun was developed from the primary armament of the preceding M10 tank destroyer though improved. The M1A2 version main gun was identified by its muzzle brake which the M1A1 lacked. The machine gun was suitable for engaging (or suppressing) enemy infantry, light armored vehicles and low-flying attack aircraft. 45 x 76.2mm projectiles were carried aboard and these could be a mix of Armor-Piercing (AP) and High-Explosive (HE) ammunition types to counter just about any battlefield need. Armor protection was between 6mm and 25mm in thickness. The vehicle stood at 7 feet, 9 inches tall and weighed 16.76 tons.
Amazingly, the M18 Hellcat was still in operational service with the Yugoslavian Army in the early 1990s - either indicating the excellent design of the M18 Hellcat series or lack of modernization on the part of Yugoslavian authorities. These M18s saw combat by Serbia in the Yugoslav Wars that followed. Taiwan became another post-war user of the type as did the rebuilding "West" German Army. American Army M18s were in service from 1944 to 1957.