The weapon was crewed by a two-man team, the Gunner and the Loader, who fired it from a prone or kneeling position. It could also be awkwardly carried, fired from the shoulder and reloaded by one man in an emergency, fired prone from the extended T3 monopod and bipod, or fired from a fixed position on a cradle mounted on the M1917A1 machinegun tripod. The weapon was carried in a T27 Cover with two padded shoulder straps, designed to be simultaneously carried by two men in line with the straps slung over their shoulder on one side.
Ammunition was packed four shells to a wooden crate, each crate weighing about 40 lbs and had a volume of 0.86 cubic feet. Three 57mm Recoilless Rifle shells could be carried per M6 Rocket Bag and slung on one shoulder by the ammunition bearers assigned to the weapon.
World War II
The first fifty production M18 57 mm cannons and ammunition were rushed from the factories to the European Theater in March 1945. Further examples were subsequently sent to the Pacific Theater. The first combat the new cannon saw was with the U.S. Army's 17th Airborne Division near Essen, Germany. While impressed with the performance of the high explosive (HE) warhead, the M18's 57 mm HEAT round proved to be a disappointment, with only 63.5 mm (2.5-in.) of armor penetration at 90 degrees, compared with the older M1A1 Bazooka which had a nominal penetration of nearly 120 mm.
In the Pacific Theater, the new lightweight 57 mm cannon was an absolute success as "pocket artillery" for the soldiers of U.S. Army infantry units that were issued the M18. It was first used in the Pacific Theater during the Battle of Okinawa on June 9, 1945, and proved with its HE and WP rounds it was the perfect weapon for the hard fighting that took place against the dug-in Japanese in the hills of that island. The only complaint the U.S. Army had was the lack of sufficient 57 mm ammunition for the M18.
Each U.S. rifle company in the Korean War was authorized three M18 recoilless rifles. Veterans of the Korean War have mentioned the use of the M18 against enemy machine gun nests. For anti-tank tasks, however, the M18 was too weak; the Soviet-built T-34 tank was extremely hard to penetrate even for the 2.36 inch (60 mm) Bazooka of World War II which had almost no chance to penetrate it. The only way would have been flank or rear shots. U.S. infantry, initially with very few capabilities against these targets, solved the problem with new weapons like the 3.5 inch (89 mm) M20 Super Bazooka, which was powerful enough to destroy T-34s.
Although obsolete as an anti-tank weapon, the M18 was still used by ARVN and it's allied forces in an anti-personnel role. It was able to use the NATO-standard M74 tripod.
US Navy Swift Boats used paired mounts containing triple-mounted M18s and a .50 M2HB Machine gun on the bow. It was used for fire support or demolishing obstacles.