A machine gun. What is the practicality in owning a machine gun? The better question is: why do I need a practical reason to own a machine gun? A machine gun is dangerous, but a car is dangerous. If I collect firearms, can the government prevent me from owning one? Should the government limit what I can or cannot own? Most profoundly, the answer is no as reiterated through the Constitution and James Madison.
The Fourteenth Amendment clearly states that no man shall "... be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use without just compensation." What is property? James Madison stated in The Founder's Constitution, Chapter 16, Document 23, that "a man's land, or merchandize, or money is called his property." In these concise lines, a founding father defines what property is. Would not a firearm fall under these definitions? A man's merchandize means whatever a person buys legally. Buying a firearm on the black market, that should be illegal, but illegalizing all weaponry should not. As long as I proceed to buy an object through the proper implications, there is no reason why I should not be able to buy whatever I would like. If I can buy a car, why could I not buy a machine gun? Both implements have the possibility of killing many people, and both implements are licensed when owned.
Also, the Second Amendment states that the people have the right to keep and bear arms. This clearly indicates that the people may own guns during a time of war but also keep guns after a time of war. During the Revolution, militia was synonymous with the people and thus that etymology should be continued, even in our day and age.
So, do I need a machine gun? No. Do I want a machine gun? No. However, the point is, I want the possibility of owning a machine gun and to keep it that way. Government involvement in the people's lives should be strictly limited to the bare minimum in all aspects of the law.