I can't add much to this convo.... Kls77, you were clearly tucked up. I assume neither you nor your mate had a crystal ball or tarot deck to ascertain that the marshalls expected you to pass with .30g bbs?
I am absolutely for spot checking and zero tolerance toward people cheating. However, unless the site CLEARLY states their policies and procedures in this regard beforehand (on website, physical site notices and briefing), then enforcing your arbitrary rules that you have written on the back of your hand just makes you look like a bully and a wanker.
Just a thought, if your friend had passed with the marshals 0.3g ammo, would the marshal have put increasingly heavier ammo in until your friends gun failed chrono? What's the heaviest ammo you can get?