I shot a grapefruit with my .50 rifle- just one ball- to demonstrate the power of a ML rifle to a large group of Boy Scouts, and managed to hit dead center. The Grapefruit " starburst" leaving no piece larger than 1/2 x 1 1/4" long. A single .50 cal. RB blows a huge hole out the backside of a watermelon. Several RBs will completely explode the watermelon, if you center the pattern.
I think a better demonstration , that leaves far less mess to deal with, would be gal. plastic jugs filled with water. If you want to get really exciting, fill a gal. jug with kerosene, and then put one of those exploding targets taped to the front of the jug. Hopefully, one of the balls will hit the target, and the explosion will set off a huge fireball and blast. Warning: the heat will be felt 50 yards away, and the shock of feeling it will take your breath away. Its one thing to volunteer to shoot the jug from 25-30 yards, standing in front of a crowd. Its quite another to be one of the crowd and not knowing what is coming. You won't be burned, but you will be checking the mirror for singed eyebrows.( not)