A second, much more radical version of the M1895 was introduced in 1917 for tank and aircraft use, later designated the Marlin M1917 and M1918, with a Swedish armaments designer named Carl Gustave Swebilius responsible for the improvements. Like the Navy Marlins, these variants used a linear gas piston in place of the 'potato digger' arm and bore little outward physical resemblance to the basic "digger" design. The new reciprocating piston was located parallel to the barrel and below it, allowing the gun to be mounted lower to the ground. Another improvement was the addition of a breech-to-muzzle aluminum heat sink, with lengthwise fins and covering some 270° of the gun barrel's outer circumference's upper areas, to dissipate heat. Most Marlin M1917 and M1918 guns saw use in aircraft as defensive armament—however, as they retained the original M1895 "potato digger" ordnance's closed bolt firing cycle, these Marlin guns, weighing only some 25 pounds (11.34 kg) apiece, versus the standard Vickers gun's 33 pound (15 kg) figure for aviation use, could readily be used for forward-firing offensive armament with fighter aircraft, when used with synchronization gear for safely firing forward through a spinning propeller - the non-synchronizable, open bolt-firing cycle Lewis gun was still slightly heavier than the Marlin, at about 28 pounds (12.7 kg). By the last months of World War I, almost 50% of the SPAD XIII fighter aircraft used by the United States Army Air Service in France had their Vickers guns replaced with Marlins. Had the war lasted into 1919 the Marlin would have been the primary U.S. tank and aircraft gun. The M1917/1918 also equipped Thomas Morse Scout aircraft used for advanced training at stateside bases.