[lurkingknight]

too much air isn't necessarily bad.. it creates certain effects that you may or may not want.

A couple being more noise at the barrel end.. it will pop louder and something called joule creep. That's something that's been explained thoroughly elsewhere. To paraphrase it.. there will be more energy to move heavier rounds, so you can raise muzzle energy using heavier rounds while keeping muzzle energy on lighter rounds lower.