Though I've never done specifically that, I don't think it would make a squat of difference.
I've launched several, well.......many
about 1/8th X 2" brass tubes down a 338 barrel without any adverse affects whatsoever.
Also with the 0.012" air space around the .284 bullet there would be a probable cushion around the bullet???
Next time he does it tell him to use try a paper patch. It may help.