"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."
Now why would they feel it was necessary to include such a statement? Part of it was Cromwell who usurped power in England for 10 years. So in the infinite wisdom of the authors of the Bill of Rights a militia system existed throughout the Confederation of States (it wasn't The United States at this point) and none of the States had a standing military. Cromwell was of course part of a standing military and instituted for all intents and purposes instituted a coup. Needless to say they were afraid of a coup happening. The organization a military unit brings would make easier to achieve. Of course a militia is a decentralized military structure except during time of conflict.
Normally, if you were an adult male you had a musket at home and when the mustering call went out you brought your weapon with you. Once there you would drill they would provide powder and shot but not the weapon you would be using during the conflict. It wasn't until later that the first armories was built for national defense.
There was also a second issue. You needed to protect your own home. In many places it could be a while for help to reach you if something went down, sometimes days. It isn't like you could call 9-1-1 and have help within 30 minutes or so. There are numerous accounts I've read where everyone on a Kentucky farm was killed and those who came to help found only dead bodies. Of course, it didn't anyone's minds that those who were killed were all armed but that's beside the point here. At least you would have a chance to protect your home.