Exactly--If the guy "got 10 years" it was not because he used a .458 Win Mag instead of a 9mm. It's because shooting someone to keep them from breaking into your car is NOT a justified shooting. (I'm assuming the wielder of the .458 wasn't in the car at the time) If someone breaks down the door of your house, emptying your 500 S&W into him is no different than taking one shot with a .22. If any prosecutor says it is, and your lawyer doesn't object, you've got a lousy lawyer.
That said, regardless of how many times I read the original post, a 500 S&W is way too loud to shoot indoors without hearing protection. There are other calibers that will get the job done and not leave you with permanent hearing loss.