On Okinawa new weapons systems and devices utilizing the latest wartime technologies were field tested and employed for the the first time in combat against the Empire of Japan. Among these were the highly effective 57mm M18 and 75mm M20 anti-tank recoilless rifles.
Weighing only 45 pounds and 61 inches long, the 57-mm. can be fired from the shoulder as accurately as the Garand. It hurls a 2 3/4-lb. shell 2 1/2 miles. The kickless 75 can toss a 14-pound, high-explosive shell more than four miles. It is 82 inches long, weighs 110 pounds, and is fired off a machine-gun tripod. It has both a telescopic and a leaf sight. 
The official US Army history of the Okinawa Campaign states:
Equipment issued to the troops included weapons and instruments of war never before used against the Japanese. New-type flame-thrower tanks, with an increased effective range and a larger fuel capacity, were available for the invasion. Each division was issued 110 sniper scopes and 140 snooper scopes, devices for seeing in the dark by means of infrared radiation; the former were mounted on carbines and permitted accurate night firing, while the latter were hand-held mounts and could be used for night observation and signaling. Army artillery and antiaircraft units used proximity (VT) fuzes over land areas for the first time in the Pacific. During the campaign tests were conducted with a new mortar-loading device, the Sound Locator Set GR-6, and the 57-mm. and 75-mm. recoilless rifles and 4.2-inch recoilless mortars. 
Known as “pocket artillery” because they could be easily man-packed, recoilless rifles saw combat late in WW2 and during the Korean War. Although late to the game in WW2, recoilless rifles were greatly praised by GIs fighting on Okinawa – with their high explosive and white phosphorus rounds they proved to be ideal and vital weapons for busting the impenetrable network of Japanese pillboxes, caves and subterranean defenses on the island.
The September 1945 issue of Popular Science Monthly featured an article on recoilless rifles and their operation titled, New Kickless Cannon for GI’s: Two-Man Recoilless Guns Give Infantry the Firepower of Field Artillery. Some extracts from the article concerning the use of recoilless rifles on Okinawa:
After 600 years, armament experts finally have found a way to take the “kick” out of gunfire. The discovery has put into the hands of American soldiers in the Pacific artillery that can be carried ashore on men’s shoulders in amphibious operations.
Kickless, or recoilless, guns helped root the Japs out of Okinawa. For certain operations, a few infantrymen can lay down as heavy a curtain of fire as a whole battery of medium artillery. They can do it because the Ordinance Department has annulled, for all practical purposes, Isaac Newton’s third law of motion: For every action there is an equal and opposite reaction.
Here are a few samples of what recoilless artillery has accomplished:
…On Okinawa, one shot from a gun of the same caliber (75-mm) destroyed a camouflaged machine-gun nest and killed two snipers in it at a range of 400 yards.
At a range of three quarters of a mille, 11 rounds from a 75-mm. scored two hits on each of two caves concealing Jap artillery. All the Jap gunners were killed. At a range of 1 1/2 miles, seven shots from a 75 scored three hits in a cave mouth measuring five by two feet.
…Against the Japanese, American troops are using two calibers of kickless guns, 57 and 75 mm. Each can be transported from ship to shore or across practically any kind of terrain by two men. One man, in fact, can carry the 57 if need be, and fire it. Usually, however, it is handled by a two-man crew – a gunner who carries and fires the weapon and a loader who lugs the ammunition and loads it. Under normal combat conditions the loader can carry six rounds, or 30 pounds. The 75, too, can be loaded and fired by two men, but in many situations the heavier ammunition – each round weighs 14.7 pounds – would make the assignment of additional ammunition carriers necessary.
The 57 can be fired from the standing, sitting, and prone positions, or it can be rested on the standard .30 caliber machine-gun tripod. The 75 normally is fired from a tripod, but in an emergency can be fired from a mound of earth.
Three kinds of ammunition are being used: high-explosive, for use against personnel, unarmored vehicles, and such targets as light field defenses; high-explosive antitank shells for use against tanks, pillboxes, and the like; and white phosphorus, good both for smoke and against personnel. The high-explosive projectiles have very sensitive fuses. They will detonate on touching foliage. That makes the recoilless gun an effective weapon against tree-sitting snipers.
…But as a way of boosting the firepower of the infantry, its success has been unquestioned. One excited Jap officer on Okinawa, practically catapulted into the arms of his enemies by gunfire, jabbered away at the interpreter, demanding apparently that he be shown something. The interpreter and the intelligence officer finally gathered what he wanted. He was interested in only one thing – the new American artillery that popped up anywhere and everywhere on the battlefield and broke all the rules of heavy-gun mobility. 
In the below photographs taken during the Okinawa Campaign, GIs of the 27th Infantry Division employ tripod mounted recoilless rifles. The M1917A1 tripod was used with both the M18 and M20 recoilless rifles.
Snapshots of GIs posing with recoilless rifle on Okinawa:
- Francis, Devon. New Kickless Cannon for GI’s: Two-Man Recoilless Guns Give Infantry the Firepower of Field Artillery. Popular Science Monthly (September 1945): p. 84.
- Appleman, Roy E., James M. Burns, Russell A. Gugeler and John Stevens. Okinawa: The Last Battle. Historical Division Department of the Army, Washington, D.C., 1948. (p. 38).
- Francis, Devon. New Kickless Cannon for GI’s: Two-Man Recoilless Guns Give Infantry the Firepower of Field Artillery. Popular Science Monthly (September 1945): pp. 84-85, 87, 238.