The Second Amendment was adopted on December 15, 1791. It states that "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." USA being more civilized and having more law & order compared to 18th century, there is no need for civilians (non militia) to carry guns.
With the evolution in firearms (the guns), the guns have become more dangerous. Guns are more capable of mass killings compared to year 1791. With time, old laws need to be revised. We live in 21st century. It is time to revise 2nd Amendment to prohibit civilians from carrying guns or any kind of firearms. Civilians can carry other types of arms, for example - swords, if they feel insecure.