We measure speeds with .20... your friend should do some research. Shooting 500 with .28s could very well destroy the internals of a BA even if it is a real VSR-10 not a PoS clone. Let alone its impact force could be substanial. You need to stop doing leg work for your friend.
Watch this. This is 570 FPS w/ .20
Airsoft Injury - unlucky can happen anytime - YouTube