Player characters from our world transported to Mythic Europe were outside the line style for Ars Magica 5th edition. I tried to put them into Grogs, as a type of Time-lost Witness, but it was (rightly) pointed out Mythic Europe cannot evolve into our world without massive cosmological changes.That being said: if you don’t care, you can easily add characters from our modern day to the Ars Magica setting.
Eric Flint, in discussing his Ring of Fire series, conjectures that every modern person in a bootstrap fantasy knows something that could make them rich in the past. His own travellers have the advantage that they moved in a group, with a large number of modern machines. It may be that the magi, not the time-lost, are the exception in their community. Lacking access to magic or machine tools, it is difficult to generate spare parts for pieces of technology as they wear out.
Communities which fall back in time find their technology level stabilising lower, based on their available resources. With the assistance of magic, however, machines can be respaired and even improved. Much as a magus making an elephant need not know how many toes there are on an elephant, so a magus making a steam locomotive doesn’t really need to know how it works. Magic makes good the lack.
Individuals lost on their own, once they can communicate may join covenants in a variety of roles. For example, an autocrat’s main skills are reading, book-keeping and managing people. You could run a great game where a modern temp agency lunch room fell back in time. I once played a great game where my Edwardian butler became a priest and kingmaker in ancient Egypt.,
General Minor Flaw: Worthless Abilities A time-lost character must have about 30 experience points which are rendered valueless by their shift in time. For example, she may have (Area) Lore of 3 for a city that no longer exists, or her Etiquette score may have been rendered useless by changing social structures. Troupes may permit companions to purchase this Flaw multiple times. This can include language Abilities, which are often rendered worthless by time travel. Delusions, that they are dreaming or can return to their own time, are also common.
I’d just treat guns as magic items with a limited number of uses. Frankly, in a game where people can cast Incantation of Lightning, I’m not too fussed about a Glock. You might, instead, say that primers and powder don’t expand explosively in Mythic Europe.