When looking at the development of history in Ars Magica, I see the setting a bit like the primary setting in His Dark Materials (you know, the trilogy by Phillip Pullman). You basically have our world, reality in this world functions a bit different. The map looks a bit different (there are no land, only boiling water, around the Equator, for instance). But historical people exist, and history will "try to keep up", conforming to these different laws of reality (magic exists and so on). History might come out differently in some places, but the world will basically be the same as in our own, real-world history.
I've been over and over the bit about Metaphysics in Art & Academe. Momentum might not exist, but this doesn't limit reality too much, or make it different from ours. In Mythic Europe, everything in reality is composed of partices, if I understand it right. These are either hot and dry, hot and moist, cold and dry or cold and moist. All these particles offers resistance to each other, and the colder and dryer the particles are, the faster it will be able to move through particles that are "lighter" (more hot and moist). Also, the more particles something has to move through, the slower it goes through it. That's why, if you throw a huge flat stone in the water, with the flat side first, it will sink slower than a small stone. But it will sink. Also, that's why a metal-tipped arrow is more deadly than an arrow with a tip of wood. And it's also why a chain mail is much more effective than leather armor to stop such arrows. It all depends on the interaction between particles. An arrow shot by a bow is given much more strength to move the particles it needs to move to create the "vacium". And this doesn't always mean the air. It means the leather of in the armor of the target, the target's flesh.. and if the arrow has more force left, it can even move through it's target and so on. That's my intepretation of it.
But what does this mean? Nothing much, really. Things in Mythic Europe acts much the same as in our real world. It's just explained in a different way. This different way might be correct in Mythic Europe, but the arrow will still fly off the bow and armor will still protect the target from it, depending on how many cold and dry particles the armor is made of. A stronger man will throw a spear harder and it will fly further, but not because of the momentum, but because of the spear's greater ability to overcome the resistance of partices on its flight, be it air, leather, metal or flesh. This is how I read that bit in Art & Academe, at least.
But how about bigger, better armor? That's easy. Plate mail, for instance, is made of even more dry and cold particles, so you need a very dry and cold weapon with alot of force behind it (remember, force is not momentum, but the ability to move through such particles) to countering it's resistance and hurt someone wearing it. And how to explain gunpowder? Well, that has been discussed before, but in my campaign, I'm considering making it an alchemical or magical substance that mimics certain Flambeau spells. How the mundanes got their hands on this? Well, that's another story. But safe to say, some rogue Veditius is sitting somewhere with an evil grin on his face.
Now wouldn't it be cool to have a book, or even a chapter a book that helps the Storyguide with longer campaigns, and what the future will be like in Mythic Europe. What kind of technological wonder might be invented, and how will this interact with the metaphysics in Mythic Europe? Maybe a "Storyguide's Saga Book"?