One of two Photos mounted on cardboard matting: 75mm recoilless rifle on tripod, Sarcee (Calgary, Alberta), 1955. This is a page from a large photo album. The M20 recoilless rifle was a U.S. 75 mm caliber recoilless rifle used during the last months of WW II and extensively during the Korean War. It could be fired from an M1917A1 .30 caliber machine gun tripod, or from a vehicle mount, typically a Jeep. Its shaped charge warhead, also known as HEAT, was capable of penetrating 100 mm of armor. Although the weapon proved ineffective against the T-34 tank during the Korean War, it was used primarily as a close infantry support weapon to engage all types of targets including infantry and lightly armored vehicles. The M20 proved useful against pillboxes and other types of field fortifications. They were phased out after being replaced by wire guided missiles, which were introduced during the Vietnam War in the 1960s and 1970s. Currently, M20 recoilless rifles are being used as part of the avalanche control system used by the U.S. National Forest Service and National Park Service.