I originally started to tap out an answer, but it quickly mushroomed into a very lengthy dissertation, and realized it deserved its own blog post. So here ya go...
Up front it must be pointed out that there are a few articles in Interplay and Space Gamer that touch on this, and some variant rules posted online. Specifically:
- There are at least two TFT variants for Traveller floating around - an older one by Ty Beard and a more recent "The Traveller Trip"
- Ty Beard also created a "HiTek" rules variant that covered modern weapons
- Both Space Gamer (No. 27) and Dragon (No. 41) magazines had articles with rules for Medieval and Early Modern firearms; not directly related to modern or sci-fi but worth knowing about.
- Space Gamer No. 65 had an article "Superheroes in TFT" by Ronald Pehr that, in addition to the title subject, also included rules for modern weapons
- Interplay No. 2 had an article by Fred Askew "TFT: Wild West" which has material for that period, to include firearms. Some technical errors but generally good.
- Chad Brandt provided an article on Sci-Fi TFT in Interplay No. 4, "Martian Vanguard Class MRAV"
- Finally, if you can find them, in the fanzine Fantasy Forum there were a few articles on TFT/Barsoom. I've never seen them, nor do I expect to - if you can get ahold of these articles I'd love to get copies!
Another point is figuring out how magic (i.e. fantasy sorcery) and technology (i.e. "gadget sorcery") interact. I touched on this in my Mnoren post - a good example is how exactly does the Invisibility spell work? Does it only affect visible light, or does it affect all spectrums, to include say infra-red and thus defeating all NVA and FLIR type technology? Of course, seismic sensors would be able to detect an invisible figure. But what about the IQ 15 Unnoticeabilty spell? What are the limits of that? The spell makes sense as written versus figures in the immediate vicinity (i.e. that can directly see/hear) of a figure using the spell, but what about someone many yards (or even MILES) away in a remote security room, viewing a security camera feed? Are they affected by this spell, or would they have normal odds of spotting the spell user in the video? I would rule that such remote viewers would not be affected, but one could argue the other way. Though the spell wording leans somewhat towards my interpretation, it does not irrefutably prove my conclusion. Note that how one rules on this would impact remote viewing via Crystal Ball in exactly the same manner.
How about the IQ 9 Darkness spell. Can it turn off a flashlight? Probably, but what happens if you simply turn it back on? What about IQ 10 Shadow - will NVG/FLIR see into that, like Mage Sight? Probably, I expect.
What effect would a Lock/Knock spell have on modern key card entry doors? Probably depends on how many features are associated with it. If it is just a card swipe, then one Knock spell will do it. If there is a keycode in addition to the swipe, or retinal scan, or whatever, then add one Knock spell for each additional feature required to open.
What about the IQ 11 Reverse Missiles spell? In a normal quasi-Medieval environment posited by TFT, a single arrow or hurled weapon coming back at the one who shot/threw it is disconcerting and a bit dangerous, but not unbalancing. But what happens when an unsuspecting mook lets loose a burst from a Tommy Gun at a figure protected by this spell? To some extent it is only fair, given what the burst would do to the unprotected target, but still something to think about. And what are the limits of such a spell? Is there a mass or kinetic energy limit to the size or power of projectile that can be reversed? Could it stop an incoming RPG? How about incoming anti-tank or artillery fire? Trebuchet or ballista shots? Bombards? ICBMs? Shrapnel from a nearby shell burst? Hypervelocity projectiles? Could one cast the spell on a vehicle rather than a figure? Imagine a Sherman tank with this spell - Tiger tanks would be knocking themselves out trying to stop it...
Can Reverse Missiles work on a laser beam? Most likely not, but the spell DOES work on lightning bolts. Even if the spell does not reverse a beam, would wizards in a tech world that have to deal with lasers then develop a Reverse Beam spell to deal with this sort of thing?
What about Images/Illusions? To a modern person a truly fantastical image/illusion might be automatically disbelieved - dragons don't exist, after all. It would just be regarded as awesome special effects, rather than a real "thing" as such. On the flip side, a medieval/fantasy world denizen might not understand what, say, a tank is, and not really be able to process it, thus allowing it to be disbelieved as well.
Going further, can a figure from a non-magic world even disbelieve something that don't know is a "thing," assuming that said "thing" is non-fantastic? If I make an illusion of a street-thug with a knife, or a vicious dog, attacking a modern person, would they think that it was anything but real? Not knowing of magic, how could they even conceive of it as being illusionary? Maybe a 4/IQ roll to "accidentally" disbelieve? I don't know - I'm open to suggestions.
How do Reveal/Conceal spells work against FLIR, MRI, ground penetrating radar, etc.?
Will IQ 13 Sticky Floor slow down or stop a vehicle?
Will IQ 14 Glamor be penetrated by looking at subject through a video clip, a photo, FLIR, etc.? Note that the use of Mage Sight allows a 4/IQ check to see through the spell when first encountered.
Would a figure under an IQ 17 Insubstantiality spell be affected by, say, radiation? Note that Thrown spells can affect such a figure.
What are the effects of lightning and fireball, in particular, against vehicles, especially armored ones? Probably minimal, though that depends on where it hits (igniting a fuel tank would be productive, or perhaps the intake of super heated air into an engine might stall or disable said component. But simply hitting armor plate would be completely useless, since the spells likely lack any meaningful penetrative ability).
Will Monster Summonings work in a modern world? Elemental and demon summonings? Probably, but something to think about.
Will a Cleansing spell cure cancer? Probably not, but one might be able to argue otherwise. Imagine how popular you could be if you could do that...
These are just a few things I've come up with off the top of my head. I know that similar topics have cropped up on the old TFT Forums talking about "industrial disease" in terms of how certain spells as written have some interesting "real world" implications if applied creatively.