I would argue that a Christian cannot serve in the military or carry a gun. I will explain why.
We, as Christians, are followers of Jesus are we not? If then we follow Jesus then we should strive to emulate him in our own lives. So then we must look at his life and teachings in order to conform our lives to that of his life.
Jesus taught that we are to love our neighbors as ourselves. He taught in his Sermon on the Mount that the old law, the old things taught, "an eye for an eye, a tooth for a tooth" but he says to love those who hate you, pray for those who persecute you. If a person comes to steal your cost give him also your shirt. If a person makes you go with him 1 mile go 2 miles. Jesus disarmed Peter when he cut off the chief priest's servant's ear and then heeled him. He never once used violence against another human, instead he went willingly and prayed for those who beat him and crucified him.
Jesus gave us a new commandment that we should, "love one another. As I have loved you so you should love one another." How does that interpret to, "I'll invade your country and bomb you because you're different than me." As Christians we are called to love even when it's hard to love that how people will know we are Christians--by the love we show.
I am a conscientious objector, that means I object and refuse to serve in the military in any form (even in a non-combatant role) because I believe that killing is wrong, sinful, evil. This includes capital punishment and abortion. I can find no Scriptural evidence that Jesus allows one human to kill another human for any reason.