There are solid foundational reasons for anyone who is a theist to call certain rights "God-given", particularly the classical trio, "life, liberty and property".
Self-defense is defense of one's life, liberty and property against the wrongful initiation of force by another.
In nature, everything that lives defends itself when attacked as best it can. Fast animals run away; sneaky animals hide; tough animals fight. Even not-so-tough animals fight when they cannot escape otherwise.
Clearly it is also instinctive in humans to fight or flee when someone initiates force against them. Clearly also it is natural and correct to do so, by almost any reasonable moral standard, when someone wrongfully tries to take your life or do terrible things to your body.
If you believe God made the world and all life, then it is natural to view self-defense as a God-given right.
In the modern world, guns are the most effective overall tool for self-defense. This is doubly true when criminals tend to go armed and are often violent in pursuit of their goals. Therefore guns are an important, if not essential, part of the fundamental right of self-defense.
Not to mention enshrined in the 2a in recognition and support of this truth.
Someone is sure to bring up "turn the other cheek" and 'thou shalt not kill'. I'll address those now.
In the day in which this was written, a blow with the hand on the cheek was a punishment delivered by a person of superior status to a person of inferior status, ie master to slave... between two men of equal status, it was an insult and a provocation to combat. As such, this references not responding to insults or provocations rather than to actual violence... notice it does NOT say "if any man wishes to ram a spear through your guts and then rape your wife, stand still and let him".... er, no.
"Thou shalt not kill" is understood by scholars of Hebrew and Judaism to mean unjust killing, not killing in all circumstances, and Christian scholars almost universally agree this is correct. Also, Hermaneutics (interpretation) requires a holistic scriptural approach, which quickly reveals there are many exceptions to the literal translation. "Thou shalt not murder" would be more accurate... murder, as in killing unjustly/unlawfully.