Is the Pater really four yards away from the entrance of the cave? I'm not sure it would have been unscathed by the HEAT round in that scenario, as it was launched just inside. I'm not sure how big the blast radius of a HEAT round is, but I'd assume it was probably up to thirty to forty feet.  

Honestly pretty hard to find information on the exact blast radius, obviously HEAT rounds aren't particularly efficient for wide-range anti-personnel work but at that proximity I'd expect the Pater would be pretty badly burned and even penetrated by fragments, particularly from the rock.