Paul says "We wrestle not against flesh and blood..." In Ephesians he goes on to tell us what kind of weapons we are equipped with in the Faith. These are spiritual weapons, since we are called to fight a spiritual battle. Flesh and blood are not our enemies, nor are we to cling so dearly to this mortal life that we use the weapons of the world to protect us.
Our life and freedom are found in Christ, and Paul says that he is convinced that nothing on earth or any spiritual force can take this from us. Not because we can 'protect ourselves' with physical weapons, but because our life is secure in Christ. He is the Resurrection and the Life.
If this is the teaching we embrace, then it is difficult to see how we can justify the use of assault weapons. This is indeed a difficult teaching to live out, but Jesus never said it would be easy, without pain or persecution. In fact, we are to expect suffering, and therefore equip ourselves spiritually and mentally to endure it while walking in faithfulness to his teachings and remembering that our battle is not against flesh and blood.