I agree with other posters, and it is not a question of a need vs. a question as to whether it is your right to own one.
The analysis of dogs and others is to the point. We have the right to own cars. It is your right to choose either a Smart Car or a Ferrari. Do you need a Ferrarri which will go three to four times the speed limit? Do you need a weapon with greater firepower? This question has no merrit. You may choose to take the Ferrari to the race track. You may choose to carry a high capacity weapon to a firing range. It is your right to choose and use in a legal manner.
Unfortunately if the law is changed, we the responsible citizens will be the only ones who comply. There will always be the few who find away around the law and/or violate the law. A perfect example are those who modify vehicles for greater power which bypass emission laws. The same can be done by those converting semi-auto to full automatic weapons.