The handgun is technically property and therefore in most states you cannot use deadly force to defend it. However in the case of a gun its complicated because it is a deadly weapon. It would be fair to say, if someone is taking your gun from you then you are in fear for your life. So, you cannot defend the property/gun but you can defend your life. I was told by several LEO that you must always remember the magic words. I only fired because I was in fear of my life/families life. You can paraphrase to "that mf'er was trying to kill me"