Being a firearms enthusiast is not like every other hobby. I don't just mean the political sensitivities - as others have pointed out, security concerns dictate that you probably don't want to blab about what you have in your house.
I never lie about what I do with my spare time - if I've got a match coming up, and somebody asks me what I'm doing this weekend, I tell 'em. But I try to remember that smart people disagree. Stupid people disagree, too, but that's another story.
If your company has a written policy that you don't get to put pictures on your PC, then you should remove the picture. If there's no such policy and you feel like your boss is harassing you, tell him and then write a note to your HR department. Otherwise, maintain your cool, answer his questions and don't get drawn into a debate. IMHO, all of this pro/anti political stuff should stay out of the office. He shouldn't try to force his world view on you, and you shouldn't try to convert him. Just do your job, go home, hug the wife and clean your guns!
JScott