"When guns are outlawed, only outlaws and governments will have guns." It seems like a pretty obvious statement, but some people just don't think things through. Making guns illegal prevents them from being in the hands of law-abiding citizens whose only use for them would be hunting and self-defense. Then the only people left with guns are the criminals and government.
This shirt should make people stop and think. Maybe a light switch will come on and they'll realize the real reason for gun control.