Mortal practitioners have problems with tech. Gard is an immortal vaklyrie. See it now?
Lifespan doesn't play into it; it's free will that matters, something mortals have in abundance. I personally doubt that Gard lacks it.
Consider also that she offers Murphy (a Pure Mortal) a job as a valkyrie. It's possible that becoming one involves losing your free will, but we simply don't know enough to say for sure.
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I don't think that dormant enchanted items cause shortout in mechanical equipment. It is the friction between free will and a mortal's magical power that causes "mana static", something not present in mindless objects, particularly those that are usable by anyone. Until the bullet goes off (and perhaps even after that point), machines will be unaffected by its magical potential (or release). Bullets are (with a few exceptions, generally either deliberate or shoddily-made, neither of which are a concern here) not damaged by the firing of the gun; they are pushed out by compressed gas, not the explosion that compresses it. Any carved runes or painted symbols or stored magical energies should be just fine right up until it hits some sucker 'twixt the eyes. Or turns into a demon mid-shot. Or explodes into a rainbow burst of magical energy, or what have you.
So yeah. If you do Maneuvers with Guns, your opponent can try to resist with Athletics or another, more appropriate skill (depending on whether they dodge the bullet or take its "effect" head on).
Spend a least few item slots on turning something into a crafting focus item for your "potions". It really is a good investment.