How do you convince parents to buy a rated mature game

Your best bet here is to formulate a good reason at why you feel that mature rating is too stringent. You understand why it was rated that way and you have a working mature knowledge on the subject matter that is restricted. For instance, if Grand Theft Auto is your game you can explain that you understand violence towards innocent people and crime is illegal and not a game in real life. Follow with the argument that you know it is just a game and it will not affect your personal life. keep in mind that they might not know what you know and might want to protect you from a subject you might have already been privy to. Discuss with them the options in a mature manner and they will more than likely let you have it. Also just point out the recreational factors about the game and hide the violence. You can say "mom you can watch me play and if you don't like it you can return it." more than likely she will not watch it.