The M18 recoilless rifle is a breach-loaded, anti-tank weapon featured in Medal of Honor: Airborne. It was designed to fire anti-tank shells and rockets with greater accuracy than wire-guided or unguided rockets. The weapon was serviced by a two-man crew, and could be fired either from the shoulder, or with the use of a mono- or tripod.
A disassembled M18 was dropped west of a German bunker-complex near Audouville-la-Hubert, meant to be recovered by troops out of the 505th Parachute Infantry Regiment to aid them against German armor. When the 82nd Airborne approaches a bombed out village, a Tiger I tank attacks the small platoon. Boyd Travers successfully eliminates it with the M18 dropped on their position. It is incredibly powerful, doing 600 damage, as well as having a massive lethality radius of 1000 in-game units. The M18 replaces the M9A1 Bazooka, the standard anti-tank weapon used by American soldiers in Medal of Honor Airborne. The only other mission it appears in is Flying Through Hail as a hidden weapon on the final level of a house on the North-West of the Flak Tower.
In Neptune, if you're in a pillbox and still have shells in the M18, and you fire a rocket on an enemy while he's on an MG42, the shot won't kill him.
The M18 Recoilless Rifle making an appearance in Normandy is anachronistic; it was not issued en masse until late 1944 and in 1945. On D-Day, the anti-tank weapon issued to Paratroopers was the M1A1 Bazooka.
The M18, along with it's larger cousin the M20 75mm Recoilless Rifle was used extensively in the Korean War.