And in protecting it in the way they are, they are of course, contributing to the erosion of your rights in other quarters.
Examples? I see no reason we have to pick and choose which rights to protect.
It's sartorial nonsense as far as protecting liberty goes. After a moments thought, it's obvious why - shooting someone is illegal. If you shoot a public official, the legality of your gun and you carrying that gun is irrelevant. There is no way for you to exercise your right to a gun in a way that protects the erosion of the central liberties.
You're conflating two different uses of the right. One is defense of the lives of self and others. I carry a handgun on a daily basis, but have no intention of every shooting a public official (unless that official happens to be illegally and imminently threatening someone's life and that's the only way I can stop it -- but that would be a legally justifiable shooting). For defense against tyranny my little 9mm (or .380 pocket pistol) is useless. My rifles, however, are not.
As for the expected riposte about how semi-automatic rifles are also useless against machine guns, cannon, attack aircraft, helicopters, tanks, JDAMs and nuclear weapons other than to say that if you think rifles aren't effective against them you need to (a) study the history of guerrilla warfare and (b) think about the political aspects of armed resistance and how the members of the police and armed forces are likely to respond to being asked to fire upon their countrymen. If necessary, consult with a few policemen and soldiers to clarify any uncertainty you may have about (b).
The reason I carry a handgun is the same reason police officers carry a handgun, for self-defense. Handguns are defensive weapons. Rifles are offensive weapons, which is why they're carried by soldiers. Oh, and before you tell me I have no idea what I'm talking about, I should probably also mention I'm a former police officer and a former soldier and a current (part-time) firearms instructor.