Would that include the right to heavy armor and munitions, or does that too sound arbitrary? I think there is something said for the argument to the absurd when what is wanted is the right, not just to self-defense, but to have an equally absurd ability to release dozens, and even hundreds of rounds in what is called self-defense. The question is not whether any proposal is arbitrary, but is there a principle whereby individuals can defend themselves with less than the military carries.
Self-defense is a natural right. How that is defined is not, and really makes no sense to think in those terms when it comes to specifics. So in addition to this principle, the Catholic Church through her bishops is giving other principles, such as the safety of the common good, that must also be considered.
It is only my opinion, but I do not consider the limitation on firepower arbitrary. I can give a reason for it. Self-defense should be limited to what is likely to happen, and that in which the actions of self-defense is not going to call more evil than good. If a man shoots at you, or threatens, then you would have the right to shoot back. If he did so in a room full of children, you would not have the right to either toss a grenade, or spray the whole room with lead in self-defense.
Sure, prepare for defense, but to define this as the ability to repel an army or a zombie apocalypse is not reasonable, in my opinion.