[sarcastro]

From what I gather you're reason for the glass breaking question is to gauge if it is going to hurt you when you get hit, and get an idea of what it will feel like.
If you want to know what it feels like (how much it hurts or doesn't) to be hit with a BB you really just have to be shot with one. Everyone has a different thresh-hold for pain what may be nothing for one person may feel very painful to someone else. There are just to many factors involved, fps of the gun, BB weight, distance, angle, where you're hit etc.

Honestly and I'm not trying to be a smartass here, if you and your friends are that concerned about it perhaps airsoft is not for you and you should look at something like laser tag.