Domestic enemies pose the real need
As has been mentioned, we have our Army, Navy, Air Force, and Coast Guard to defend us from "foreign" enemies. The reason for the Second Amendment is also and now more importantly to defend our freedoms from "domestic enemies." Those who propose to strip guns from the civilian population should first consider the potential consequences. I believe it was the leaders of Russia, China and then Germany who first disarmed their citizens by making possession of guns illegal and then confiscating those guns before declaring themselves dictator.
The reason for civilians to possess semi-automatic weapons, and yes, high-capacity clips for those weapons, should be obvious. Every time someone buys a high capacity modern sporting rifle, (which some like to refer to as military style assault rifles for the stigma the name carries) it is an insurance premium they are willing to pay to defend the freedoms we enjoy. "There are more instances of the abridgment of the freedom of the people ... by the gradual ... silent encroachments ... made by those having political power ... then by violent and sudden usurpations." James Madison
If you are against gun ownership by civilians, I suggest you put a sign in your front window that reads: "There are no guns in this house." Are you willing to do that?
We need to try to stop these mass killings, but it should be by addressing the cause not the means by which these acts are carried out.