Combat will be handled in much the same way as it was in the Middle Ages. Modern firearms don't really make that much difference to the Missile Fire rules. There's still Between Time, Movement, Missiles, Melee, and Magic. There's still First Stike, Attack, Defense, Damage and Soak. Bear in mind, though, that normal-sized human characters only have those 5 Body Levels, and modern weapons are often much more efficient at taking them away.
Modern HTH weapons will be similar to the ones currently on the chart, though exotic weapons like nunchuku may require some "tweaking." The Missile Weapon Chart will now include guns. They will tend to have a high Expense, Initiative, Attack, and Damage, good Range, with no Str minimum or Load penalties. New rules will be incorporated to cover automatic weapons, shotguns, grenades, etc. The upside to this is that the PCs have the opportunity to use these things. The downside, of course, is that they also have the opportunity to be used against them. Combat can very quickly become deadly.
Given modern knowledge of the human body and widespread awareness of antiseptic techniques and the like, wound recovery is handled somewhat differently. First Aid substitutes for Chirurgy in the field. A First Aid + Int + Wound Penalty roll of 9+ gains the wounded a Body Level, 3-8 merely stabilizes the wound. Botches cause the loss of a Body Level for each Botch. Then the Wound Recovery Chart is consulted. The result is the number of days it takes to recover that particular Body Level if left under these conditions (superior care or worsened conditions warrant another roll, which cannot be better in the latter case). The result is halved if the wounded person makes a stress Stm roll of 6+, excluding wound penalties. If Medicine is used instead, substitute that for First Aid. The result on the chart is halved automatically, then halved again if the above Stm roll is made. In either case, a new recovery and a new Stm roll must be made at each Body Level. X means a roll on the Catastrophe Subchart; a Botch means two rolls, plus one for each Botch beyond the first (i.e., three rolls for a double-botch, four for a triple, etc.). Furthermore, you automatically recover that Body Level at the slowest rate listed with no modifiers (i.e., 14, 60, or 80 days).
Surprisingly, magic and its use does not change much. The Arts, Formulaic Spells, Certamen, Mastering, Parma Magica, Foci, vis, etc. are all being left alone. The limits of "Hermetic" magic are still in effect. The big differences occur in Spontaneous Magic and what you can cast spells on.
When casting Spontaneous spells, you still divide your total by two when exerting yourself. However, since magic is unpredictable, there is a chance that your total may vary when not exerting yourself. The Storyguide will roll a secret six-sider. On a 1-3, the total is divided by 5 as normal. On a 4-5, the total is divided by 4. On a 6, it is divided by 3. You won't know the final total except by the effects. This might help you out in a really tight pinch, or when you want to conserve your energy.
As to what targets can be effected, there are no longer any restrictions that prohibit casting spells on another mage or, for that matter, anyone (no Order of Hermes). However, in today's modern society, secrecy is still vital to most magic-wielders. Rumors abound that if someone is becoming too obvious, they are quietly eliminated and their antics dismissed as special effects. Thus, it's probably not a good idea to reveal yourself too often to the mundanes (at least not in such a fashion that it can't be "logically" explained afterward).
Furthermore, there are certain materials today that simply didn't exist in the Middle Ages. What Form does one use to affect plastic? The answer is that these synthetic materials — plastic, nylon, styrofoam, polyester, etc. — are inherently non-magical. Magic is the ultimate "natural" force. Being un-natural, there is no correspondence to these things in the Realm of Forms. Magic cannot directly affect these kinds of substances.
A quick side note on rubber: natural rubber and latex are simple plant products, and can be affected by Herbam spells. However, between the late 1930's and the present day, these have been almost completely supplanted by less expensive synthetic versions. It's going to be up to the Storyguide to determine if a particular target is affected.
Synthetics are used by many experienced modern mages to contain or block mystical energies. However, while these things may not be affected directly, indirect means may suffice. For instance, an Unseen Arm may not be able to lift a styrofoam cup, but a Broom of the Winds could blow it away, or a Pilum of Fire could burn and melt it. In general, any spell that requires a Form or a Casting Requisite for the target cannot affect synthetic materials, though almost any spell is bound to be exceedingly unpredictable when cast upon them.
As for Spells, note that the Range and Duration of most spells are in not-clearly-defined units like paces and Sunrise/Sunset. We will preserve this method, since magic, again, is often unpredictable. Also, some spells don't really need to be Ritual spells and some non-Ritual ones need to be. As a compromise, I have a list of spells that are marked as Slow spells. Slow spells are treated the same as the normal spell, except that, like Ritual spells, each Level requires three minutes of casting time. Raw vis is not required to cast the spell, but Range and/or Duration can be increased with its use as usual. If the spell was Ritual to begin with, Long-Term Fatigue Levels are involved in the casting; if not, Short-Term Levels are expended as normal.
There is no spell, such as Change the Nature of vis, which can alter the nature of Art-specific vis. That goes against the Law of Essential Nature. Furthermore, there are several other spells, too many to mention here, that will be altered or disallowed, with or without notice. This is due to several factors, such as changes from 2nd to 3rd Edition, changes from 3rd to 4th Edition, differences in opinion between the authors and myself, and the fact that the proofreaders and editors of 3rd Edition Ars Magica seem to have taken one too many drunken holidays.
Today's world moves swiftly. Thanks to technology, we have access to refinements, techniques, and materials that Medieval mages could neither imagine nor conjure with all but their most powerful spells. In standard Ars Magica, the basic unit of activity is the Season. Using their crude laboratories and hard-to-read tomes, wizards could do one "thing" every three months or so.
That has changed. The basic unit in modern Ars is the Month. There's no lab activity, to my knowledge, that a modern person cannot do in one-third the time or less. Reading and studying? We have more legible texts and are more literate. Magic item construction? Better tools and materials. Potions? More accurate measures and pure ingredients.
Because of the change, certain Virtues and Flaws that have effects per Season or Year may also change (others may not--Storyguide discretion). vis gathering and creation falls into this category. Magical energy in the modern world is weak, and modern laboratory techniques can only do so much to counter this. When distilling vis from personal source or a magical aura you can get 2 pawns every three months. Spells will tend to have the same Durations, unless I spot some glaring contradiction.
You may study Summae, Lab Texts, Grimoires, etc. in a Month only if 1) it is in modern typesetting (or if the handwriting is particularly good), and 2) if you have a Scribe <whatever language the text is in> of 4 or more. If either of these requirements are not met, the task takes two Months. If both are not met, it takes three Months (i.e. due to the conditions, you're back in the old-fashioned Season). You may spend time copying a handwritten text (to either a handwritten, typed, or computer-based medium), or translate one into modern English (or whatever your native tongue is) as per the rules in 4th Edition. From then on, the text takes the usual Month to study. Note that even then, some writings (other wizards' spellbooks, lab texts, etc.) may still require further translation to get it into a useable form you can understand (see rules).
You will not automatically know the score (or even necessarily the topic) of a written source. It takes (ideally) a full day of perusing/skimming to glean this info. If the source is less than ideal, use the above study guidelines to determine the days.
We are going to use the 4th Edtion version of most Laboratory Activities. It's the same as 3rd Edtion, except that 1) Magic Devices are easier to Instill Powers into, and 2) you can extract Vim vis from a Magical Aura. We will also apply 4th Edition rules when studying Magic Theory from vis, using Familiars, and brewing Potions.
Instilling Powers into a magical Device will be handled under 4th edition rules, which are faster than 3rd edition, but not as fast as 2nd. You won't be churning out legendary artifacts in a couple of Months, but you won't be spending nine years making a coat that keeps you magically warm, either.
If Distracted from your studies, you may miss up to four days in a Month with no penalties. For lab work, your Lab Total is reduced by 2 for each day beyond four. For studying summae or texts, your Study Total is reduced by one level for every day over four that you miss. If the activity you are qorking on requires a full Season/Month, and you take more than four days off, it is ruined and any vis is wasted.
Arcane Experimentation differs radically between the Editions. I'll be comparing them and taking parts that I like as I find them.
I realize that one activity in particular may still seem like it takes too long: copying spells and texts. "What about my word processor?" you ask, "And can't I just Xerox six copies for my friends?" You're right. Simply copying spells or texts (provided they are already on an appropriate medium--see below) takes whatever time is reasonable. But remember: most magi guard their spells and texts very jealously. It's no small thing to give even a buddy a computer disk with a copy of your Grimoire on it.
The modern Ability of Typing may also come into play when copying spells or texts. Propping a medieval illuminated manuscript up on the color Xerox down at Kinkos could be a problem, so Modern magi may want to convert their libraries into something a bit easier to handle.
Typing out a handwritten text is based on the rules for copying spells as listed on page 82 of the basic book. Working from shorthand notes, you can type up clear copies of (Typing x 20) levels of spells in one month. Understandable handwritten spells can be typed up at the rate of (Typing x 60) levels per month.
Similarly, copying texts with Typing is a relatively quick and painless process. Summae can be done at a rate of (Typing + Dex) x 3 Levels per month. Libri Quaestionum can usually be typed up in a week or two. If you have a Typing score of 5 or more, copying a LibQ can be done as a "distraction" from a second lab activity in the same month, provided that there are no other distractions.
Typing can frequently be used to improve the physical quality of difficult to read texts. On the other hand, a text with numerous color illustrations or complex diagrams would suffer a penalty if the only version available was a typed manuscript. Quality judgments will have to be made by the GM on a case-by-case basis.
Given the modern person's average lifespan of 70 years or more, plus the new rule setting the Month as the basic unit of activity, aging is nowhere near the important consideration that it used to be in Ars Magica. Still, people do grow old and die (especially if they have magical help), so rules need to be adapted. Aging rolls are now unnecessary until age 50+Stm, not 35+Stm. They're still only made once a year and are still affected by Longevity Potions, etc. We'll use 4th Edition aging and longevity potion rules. There is an additional -2 to the aging roll, to offset the difference in decades (i.e., so that a man of 50 today has roughly the same chances of feeling the effects of age as a man of 35 did centuries ago). Decrepitude adds to this roll and subtracts from the roll to avoid characteristic loss and death. Everyone chooses a birthday when the character is generated. The aging rolls are made on the character's birth month, secretly, by the Storyguide. The results of the roll, if any, will be felt during the subsequent year, at a time unknown to the player (so everyone doesn't suddenly start dying on his/her birthday).
Magical aging puts a bit of a twist on things. Everyone may need to keep track of as many as three different "aging" stats: Chronological Age, Actual Age, and Apparent Age. Chronological Age is the absolute amount of time the character has been alive (as determined by his/her birth certificate). Actual Age may differ if magical aging, supernatural powers and effects, or the draining effects of casting certain spells have come into play. It is this stat that determines when you roll for natural aging, not necessarily Chronological Age. Finally, there is Apparent Age. This is what age the character looks to be. As mentioned above, this can be altered through the premature use of a longevity potion, or through other supernatural means, like Faerie Blood. "Good genes" can also keep someone looking young (or older, for that matter), so there's some leeway with Apparent Age.
There is no Realm of Reason. It just doesn't make sense to me that a scientist, for instance, can cause a spell to fizzle just because of his disbelief, or that textbooks can somehow become gibberish if they happen to be blessed. Dominion will suffice to suppress magic in cities and universities. "Divine" auras will be moderate-to-high nearly everywhere in modern times. Dominion is what you get when mundanes--God's chosen creatures, created in His image--gather to live, unless they're actively evil or somesuch.
In any case, if a rule is not mentioned above, assume it is unchanged from normal 4th Edition Ars Magica. I, as Storyguide, reserve the right to alter, add, or abolish any rule or set of rules that I feel needs it.
Lastly, by all means feel free to comment and suggest, tell me what you like and don't. I'm open to feedback.