Ever since Charles Sheldon's book In His Steps popularized it, "What would Jesus do?" has been a helpful question to start thinking through ethics from Jesus' perspective. To get the answer, of course, we have to know something about Jesus. What did he do? What did he say and teach?
Those are critical first steps to working out the next question that needs to be asked, "What would Jesus have me do?"
It's too simplistic to say, "since Jesus didn't have a gun, we cannot" or "since there were no guns for Jesus to forbid, no guns are off-limits." When it comes to guns or any other issue we're grappling with, the question is, "What would Jesus have me do?" Remember Mark 8:34? The command from Jesus was "take up their cross," not "take up my cross."
Of course the cross is the common element. We can't bend the distinction between Jesus' cross and ours so far as to make them unrecognizable to each other. We aren't called to live exactly the life he lived (we couldn't for a variety of reasons, not the least of which is we live in a different place and time and the greatest of which is we're not the Son of God). We are called to live after his likeness, which means letting his life and teachings and the lives and teachings of his followers through the ages have great bearing on us.
We who call him Lord are called to live lives that take seriously who Jesus is and who he'd have us be. We must, as his followers, take seriously the cross he bore on our behalf and the cross he called us to bear on his behalf. We must ask the question more frequently and more fervently about what Jesus would have us do because until we do we'll be captives to the question that our less "disciple-ined" side prefers to ask, the one that comes much more easily to us, "What would I have me do?"