I'm curious, how have magi in your sagas gone about enchanting missile ammunition such as arrows, crossbow bolts, or sling bullets?
I'm toying mentally with the idea of making a potion that one could smear on arrow heads that would cause a Muto Terram (Ignem) effect triggered by the presence of blood for extra +X damage.
Does that seem doable under the rules to any of you, or somehow specifically prohibited? If it makes a difference we are using 4th ed. still.
The most straight-forward way is to enchant arrows as charged items, one charge per arrow. If you have a Mechanical (Rustic magus), you can do that even more efficiently.
But if you have Tethered Magic or Mutantum Magic, then you can make a few spells and put those spells right onto items like arrow.
I believe there is a section on charged items in the 4th core book. I'll take a look at that, thanks for the suggestion @callen.
See ArM5 p.96f Charged Items for ArM5.
That could certainly be done, and is far from the only way to do so. Unless you're much better at Muto than Creo though, I'd just go for straight CrIg mind you. Though either effect does mean that you're potentially spending seasons on making items that your grogs then throw away (ie, miss their attack rolls).
That should be fine. Or as @callen suggests, just enchant the arrows directly. And remember penetration!
This was actually the only part that gave me pause. Can I ask why?
I'm ... not a huge fan of the 4th edition, while I greatly enjoy the 5th (and still sometimes miss the earlier editions).
It's what I have, and am used to. As a bonus the core PDF is free so my players were able to quickly acquire copies to begin learning the rules. Second bonus, hard copies have been easy to come by cheaply via Amazon and eBay. It's also been quite easy for us to house rule into the urban fantasy game we wanted, a non-Fate quasi Dresden Files.
I find it more curious that you say you sometimes miss the earlier editions. I recall 3rd being quite controversial when it was around.
I figured as much. I can recommend the 5th edition. Strongly.
Honestly, I used to run a 2nd/3rd edition hybrid. Libraries were much simpler, and while Arts-based-on-XPs appears to be popular, I have some reservations. Still, it works (in the 5th edition anyway).
I played a Welsh giant blooded mage called Bercelak the Bowman in an old game. Verditius, used charged items as arrows, with a variety of effects, magic cancelling, bursting into fire, etc. It worked surprisingly well because he don't end up using the charged items that often. It turns out that for many foes, an 8 foot long longbow works just as well.
I'm actually thinking a house rule bending the potion rules might work better than charged items for "one shot ammo" in 4th ed. rules but I'll need to crunch some numbers to confirm if there's any real difference.
Also rethinking the original Mu Te (Ig) idea. Considering a Pe Te instead, sort of a mini Obliteration of the Metallic Barrier for the extra damage. Think "claymore mine arrowheads" or, in actuality in our modern game, "claymore mine bullets".
I could technically say the same about a 2nd/3rd hybrid, I suppose. I ran 3rd for a while, minus the Realm of Reason but using the native combat system. That's basically a mix of 2 & 3, right?
I liked the addition of the Target parameter in 4th, and the combat system was pretty easy to adapt to firearms for a modern Urban Fantasy setting.
You could also use a short duration, non-ritual version of the watching ward. cast on an arrow, then cast the spell it is to hold, then fire the arrow.
But the special, indeterminate ending is what the Ritual allows for. The guideline without the Ritual fires off the spell when the container ends. I suppose you could do it with Concentration and break that concentration when the arrow strikes, but that's a lot more difficult to pull off.
You can hold it in a Ring, traced on a fragile glass arrowhead that shatters on impact.
Of course, you probably want to hold those fragile arrows in some appropriatedly charmed quiver, or at least at a safe distance 
Maybe.
I'd be a mite worried about the exact definition of Ring.
Yes, that would work.
Canonically, you can have mobile rings on flexible surfaces that keep working.
It's the 'Inside' part that bothers me actually. But it should be OK I think.
Yes, that could be problematic. But you could probably work it out.
I'd allow a short (diameter duration) container that expires under very simple conditions. i.e. the arrow hits something. You are then loosing the ritual tag, but also the indefinite duration and the ability to tie it to complex conditions with another spell. Seems reasonable and not contradicted by the rules.
If you're inventing a formulaic version of a container, you could potentially use a non-standard duration - either a fixed length of time that's just enough to cast the next spell and then loose the arrow, or (probably harder) something that's linked to the arrow's flight.