I photographed these M40 105mm (“106mm”) Recoilless Rifle examples at Quantico and the Ordnance Facility; Fort Lee
The barrel was 105mm, but the weapon was referred to as “106mm” to prevent soldiers from trying to use 105mm shells intended for the older M27 recoilless rifle.
The M50 Ontos used a set of six M40 105mm Recoilless Rifles.