The reason the second amendment exists is because the founding fathers had just beat the best trained best equipped military in the world with a volunteer army with no training.
It has been proven time and time again that no matter how good the army is, a military force can not defeat a determined populace (example: Iraq, Vietnam)
There have been many studies that show that when you allow law abiding citizens to have guns, crime goes down.
Crime Plunges in Pro-gun Town