There are four basic reasons for something like the second ammendment.
Every one of the founding fathers is on record to the effect that private ownership of firearms, the 2'nd ammendment, is there as a final bulwark against the possibility of government going out of control. That is the most major reason for it.
At the time of the revolution and for years afterwards, there were private armies, private ownership of cannons and warships. . . The term "letters of marque, and reprisal" which you read in the constitution indicates the notion of the government issuing a sort of a hunting license to the owner of a private warship to take English or other foreign national ships on the high seas, i.e. to either capture or sink them. The idea of you or me owning a Vepr or FAL rifle with a 30-round magazine is not likely to have bothered any of those people.
The most major motivation for the present generation of gun-control laws, i.e. the problem with drug-dealers owning AKs, is a drug problem and not a gun problem. Fix the drug-problem, i.e. get rid of the insane war on drugs and pass a rational set of drug laws, and both problems will simply go away.
But I digress. . The 2'nd ammendment is there as a final bulwark against our own government going out of control. It is also there as a bulwark against any foreign invasion which our own military might not be able to stop.
Just prior to WW-II breaking out in the Pacific, a meeting took place in Tokyo in which a number of Japanese general officers asked Isoroku Yamamoto, the only one of their number to hve spent any time in the United States, what the problem was; why not simply invade the place and get it over with. Admiral Yamamoto replied that the problem was not the US military, that there were fifty million lunatics in this country who owned military style weaponry and practiced with it, and that there would be "a rifle behind every blade of grass". This apparently bothered him a great deal more than the 300,000 or so guys in uniform prior to the war. (this is same reason the Japanese were hesitant to invade mainland Australia. It was full of men & boys who could shoot accurately and were ARMED)
A third obvious reason for private ownership of firearms is to protect yourself and your family from criminals and wild animals. Criminals in fact are not the sum total of problems in the world which firearms can help in dealing with. In particular, we read about tens of thousands of people being killed every year by poisonous snakes in India; it's hard to picture that happening if the people were armed.
Finally there's a fourth reason for the 2'nd ammendment, which is to provide the people with food during bad economic times. When you listen to people from New York and from Texas talk about the depression of the 30's, you hear two totally different stories. The people in New York will tell you about people starving and eating garbage, and running around naked. The Texans will tell you that while money was scarce, they always had 22 and 30 caliber ammunition, and that they always had something to eat, even if it was just some jackrabbit.