That is another note, and since the Composite Bow thread has its hands full, I thought I'd address this here.
You're asking several things, here.
First, any "magical bow" can either launch an existing, mundane missile/arrow via Rego, or it can first create a magical projectile and then launch same, via a combination of Creo/Rego.
With the first, I believe the questions of magical resistance are ignored, since the arrow is, in and of itself, mundane. However, in the second, the projectile is quite magical, and so Penetration rules come into effect (of which others can speak better than I.)
As for what type of projectile can be created, it's only limited by imagination. Technically, a classic, standard arrow would require Herbem (the shaft), Terrem (the head, whether metal or flint), and Animal (the fletchings, feathers.) However, other spells show that Herbem by itself can create a wooden stake that serves quite as well. Since the magical missile will then disappear after the spell duration is over, it matters little if it is overtly magical in shape for better flight, or perfectly normal to mundane inspection- the fact that there is a wound with nothing within will speak for itself.
Fire? All-metal? Pure animal bone (for some obscure reason)? Sure- any of the above. Or a javelin of ice, or a concentrated blast of wind, or an anti-demon effect, etc etc - but now you're starting to talk more about a magical device with a special effect of appearing like a missile, rather than what would need to appear to be a "bow and arrow." If you're using Form/Effect bonuses, "bows" would be best for summoning/shooting normal "arrows", I'd say, and some of those other effects would be better served with some other form. For instance, a cross to destroy demons, or a scorched oaken staff with a ruby for fire, or whatever, etc. (Bows would also be highly appropriate for some Perdo Animal spells - just tossing that out.)
Aiming- that would be built into the effect, and thus the device itself. Note that devices do NOT allow for Skills to improve their accuracy, as a rule. The spell has a "+X" to hit - that's it, possibly modified by the user's Perception. William Tell uses it (Bow +10, Perc +3), or his sharp-eyed child (Bow +1, Perc +2) - not that much difference. Various bonuses could be built in by increasing the level of the effect, but practice would never improve it for any non-magus character.
Requisites, Ranges, Base Levels- you're creating a magical effect here, I'm sure you can read the Guidelines in the book as well as any. Find the effect (Rego or Rego/Creo) for the Form that you decide upon, and pick your range, etc etc. (Yes, you could launch a projectile "Sight" range- if you can see it, you can hit it. Or a "bow" that creates/shoots several arrows at once- that's magic.)
In one (3rd ed) saga I was in, a demon-hunter Companion got a mage to make him a "cross-bow" to hurt the unholy, consisting of a large cross inset with appropriate semi-precious stones (turquoise, iirc?) with a curved front to hold missiles, looking for Form to work vs undead/demons/etc. Holding it like a crossbow and setting a missile along the cross, the item would "bless" (charge w/ anti-Vim) any wooden item set within it and launch it a certain number of times/day. The problem was that no amount of practice would improve the chance to hit, and if the character ever had no appropriate bolts (which he "usually" carried), he had to improvise. But when it worked, it was a great little toy.
