@Scayde: Sorry, it went "poof." I didn't save it anywhere else.
EDIT - Here is an abridged version, drawn out of my own patented and rather unreliable memory banks.
Dimensional travel in an RPG is always an interesting subject. Here's some of my thoughts concerning Scayde and her wondrous rifle.
Scayde is from a universe (read: Prime Material Plane) where physical and mechanical laws reign supreme. The magical weave is so anemic as to be virtually non-existent: in her world, creatures respond directly to their enviroment according to these forces and laws without the influence of magical energy.
The Primes exist much like frequencies in a bandwith that has no beginning nor end, stretching on and on into virtual infinity. Each individual frequency on this infinite bandwith represents an individual bandwith that in turn is infinite unto itself, knowing no beginning nor end.
Frequencies (universes) that are closer to one another in this procession of Primes are by nature very similar to one another. Their laws closely resemble each other, as do the composition of their worlds. The closer two universes are on this vast bandwith, the more similar they are to one another. However, the farther removed two universes are on the bandwith from one another, the more they differ.
Things work differently in each individual universe - each is a subtle variation of the other. It is reasonable to assume that two adjoining universes can safely interchange matter with no effects at all. This safety buffer decreases, though, the farther the Primes are apart on the bandwith.
Scayde's universe is far removed from the Prime Material Plane which houses Toril and the Forgotten Realms. The Magical Weave is powerful in Toril's universe, melding with the physical forces and laws to create the creatures we have all come to fear and despise. Spells can alter the very fabric of space and time in Toril's dimension...whereas in Scayde's home plane, magic is feeble and weak, and often is not present at all.
Based upon this line of thinking, it is easy to see how Scayde's character would possess a high degree of immunity to spells. Both her mind, and the peculiar energy emantions of her body, would intrinsically resist magic, whether it be harmful, or beneficial. An example:
A fireball roars into the midst of the party, sending everyone scattering to the four winds. Scayde is busy trying some pretty new boots on that Yshania gave to her just a few moments ago, and wonders what all the fuss and bother is about. She's just in time to witness a pretty fireshow, and she has a front row seat as the ball of flame explodes. AC buries her head in her hands, mourning the loss of her strange new friend.
Scayde, however, wonders why she isn't the least bit toasty. Oddly enough, though, Yshania's nice boots are now nothing but ashes on her feet. Yet, her jeans, shirt, and vest are just fine. Amazing, she thinks. How about them apples?
Digging her old boots out of her saddlebag and slinging her rifle on her shoulder, she walks over to AC and asks her if she's doing ok.
Both Scayde and the items she brought with her into Faerun are little pieces of home away from home, so to speak. Originating in another universe, they emanate strange energies which intrinsically resist the Magical Weave. For a time, everything works just as it should: she can fire all of the bullets she wants, and stick her tongue out at the Drow wizards.
Eventually, however, the energies of Toril's magical weave will begin to have an effect on Scayde and her mementos from that strange world called Earth, effectively conforming her and her items to a normal state - normal, at least, for Toril.
It stands to reason that things work a little differently on Toril than they do on Earth. Gunpowder that goes "BOOM" on her daddy's ranch in Texas would more than likely go "pouf" in the dark tunnels that the DF finds themselves in. This is likely due to the fact that effective gunpowder has a different composition on Toril, due to the presence of the Magical Weave. For a time, Scayde's gunpowder resists the laws and energies of Toril, and continues to work fine. There comes a time, however, when it begins to fail, eventually reaching a point where it does not work at all.
Thus, there will come a time when Scayde will be able to cast spells, and feel a great deal of discomfort when those magic missiles hit. In fact, they'll hurt quite badly. At that time, she better run for cover when the fireball comes.
There you go, Scayde. Does that cover it?