Finally why are magical things treated this way when in other universes while a plasma gun can be taken nearly anywhere and used nearly anywhere and also (if you bring a generator)have its "charges" replenished nearly anywhere?
Because a plasma gun functions using basic, for lack of a better definition, real-world physics, which are assumed to be present in most worlds (Admit it. Our players are mostly going to be human/generally humanoid and made of matter, right?), while magic may be dependent on divine forces only present in one realm, or a type of intrinsic energy only present in one realm/several realms.
For that, it may be possible to do something akin to imbuing the rune/hermetic circle with their universe's native energy, like how the proposed cross-plane 'mana pool' would work.
This appeals more to me than having magic not work/recharge as otherwise tech becomes better as you can just lug an extra battery for your laser gun. I think that there should be some method for doing this magic storage thing so the resource is still limited but still able to function in other places.
Yeah, you can lug a battery around, but presumably not an infinite number of batteries (I'm assuming all our GMs are reasonable). An idea that occured to me is that we can allow storing home-universe energy (mana, whatever) in items (amulets and so on), from which a mage can replenish his mana while in another world. Effectively magic batteries. That's gonna bring plasma rifles and fireballs to an equal level.