As far as I know, the weapon is safe. All large cannon in a tracked armour vehicle, never allow any crew members to be directly behind or below the gun/breech block. If u see all MBTs, and Self propelled guns or tracked anti tank weapons, the weapon is loaded from the side. In other words, tank loaders usually have strong right arms as they have to carry and ram the round into the gun. If you look at the interior of the Primus SPG (and there are several Youtube vids out there), even with the gun at maximum elevation. there is a gap between the breech block (the tail end of the gun) and the floor. And also between the breech block and the back of the turret as the vid I saw was filmed in the back of the turret. Frankly, I don't know how he could have got crushed. There is enough room.