you're suggesting that plastic bb's are marshmallows by comparison. they are not. the C.O.R. is pretty close. a better choice would be
a. getting struck by a rubber mallet
b. getting struck by a rubber mallet painted with aluminum.
both are going to hurt like a bitch but the difference in damage is negligible. it's alu. coated but plastic core. try wrapping tin foil around a tennis ball. it might do a little but not worth mentioning.