This is one of the many experimental projects of the late Gerald Bull , my old buddy and father of the HARP program. He was a certificable ordnance genius.
This was an attempt to produce an “EXPLOSIVELY FORMED PROJECTILE” along the lines of the HEAT shaped charge projectile.
This .50 cal BMG case is very heavy walled and fired a pure copper disc seated inside the case near the shoulder. He tried a variety of calibers based upon easily obtainable surplus guns and barrels; .50,20mm,37mm,6pdr.
The 20mm fired the Canadian penny which at the time was very heavy copper content.
The pressures generated made it impractical for tactical development.