We're currently discussing a house rule that would see all effect modifications removed from the construction of charged devices.
I'm not too convinced but our reasoning is that a magi 20-30 years out of gauntlet (which ours are) could spend a season making a Perdo Arrow which could possibly drop a dragon or demon. Basically a single season to kill a big story bad. And if the Magi COULD do this is flies a little in the suspension of disbeleif that the wouldn't. Now this is fair comment but we are trying to make a few special pleadings for penetration such as using the manufacturers penetration at the time of casting and/or incorporating arcane connections (which would mean going through the story of getting one).
Andrew, the problem might not be in the arrow per se. The problem (as I see it) is that magi have gone to skyrocket in power level in the last editions, while their supernatural opponents (said dragons and demons) have been kept at 3rd edition power levels. So, they are basically a bunch of losers in front of the hermetic magi. Correcting the later might be easier than correcting the former in a lot of cases. Except penetration, that is just wrong in the items as written. I have introduced a Might 200 dragon recently in my saga. I am unsure if the magi will be unable to beat him on a contest of raw power, but I am trying. It is not very intelligent to keep game balance.
To avoid your bad guy being killed as soon as stepping into sight by some charged item with high penetration, you can use some "tricks"
1.) You know the hight of the penetration in that device. It will not change, so you can raise the opponents MR as high as appropriate. However, this is a bad choice for me if this leads to incredible scores in MR.
2.) Give the opponent a virtue or supernatural ability to be protected against these items. (Resistance to fire, shrug off any one damage at the cost of might points). My experience is that players put their priced charged item away as soon it does not inflict the intended damage. (Not knowing the creature might well run out of might points before the item runs out of charges.)
3.) Give them a virtue to double their MR when being targeted by a certain form or technique/form-combination. This way other players might still use their spells on them while the one with a charged item cannot.
4.) The first appearance is not made by the bad guy himself but an illusion of him, which might either take no damage or seems to "die". The bad guy might turn up later convincing the players he cannot be seriously harmed by their magic.
5.) The bad guy knows the danger that this wand poses to him, so he lets it be exchanged by a useless piece of wood, or if being a hermetic magus, by a device that fires back (though this need not be lethal). If that bad guy has the ability to find out how to use the device this might turn out as a disaster.
It all depends on the type of adversary you through at your opponent. There is afterall a reason covenants become more political as they grow older.
for example, A specialist Perdo Mage such as what I run, will run afoul of hordes of villagers upset about something, a Level 20 might will stop many spells outside the mage's specialistions. A perdo Corpus specialist is really going to have issues with a mobile tree or demonically animated fire.
It's often more about tailoring threats to the magi's weaknesses.
That said, I'd like to add a couple of my own "death arrow" workarounds:
The issue is not necessarily that you can whack THIS creature with one arrow, but what happens when you get to be known as "the guys with the magic arrows."
Every village with an adventurous lad is banging down your door, saying "save us!"
And then the noblemen who are embarrased that they can't deal with the dragon, etc., want you to help. And if you don't, then they try to turn their remaining knights on you for the arrows . . . AND THEN, regardless of the outcome, the Quaesitores launch an investigation on "interference with mundanes" grounds.
Plus, who's making these arrows? Is it a Veriditus? Is there a possibility of vendetta? Or, if you already have enemies, wouldn't they want to sabotage the arrow process to hurt your covenant? Possibly a High Crime, but if a traitorous grog switches arrows before you set off for the cursed cave, you may not come back to bring it to Tribunal.
You'd better hope your antagonists don't have supernatural arrow-catching reflexes or the ability to breathe fire so hot it destroys an arrow in flight for your Touch-range arrow.
Or, taking a page from Beowulf, the bad guy's successor (mom?) is much smarter, and either develops the above abilities, or is a big fan of stuff like labyrinths of demonic mirrors.
Okay, so your magi are invincible. So the bad guys' associates go and wipe out a Spring covenant somewhere. And look like they're threatening another. And now you have to waste what would be perfectly good study time chasing creatures that don't want to face you, but will terrorize lesser members of the Order, all over Mythic Europe, or you have to answer to the Grand Tribunal why they have to convene a small army of magi to clean up your problem.
worth noting that making these arrows is a good waste of perfectly good studytime... And if someone is massproducing this kinda wand - just start attacking with low might foes... Or tricky ones... (like invisible demons)
Or enemies that can walk in plain sight because they are not enemies that you can combat blasting them away.
One of the GREAT things about Ars magica, is that actions cause reactions: what the characters do does not fall into a void, but has consequences. This is the main difference between the narrative style of Ars magica and the more DnD/LOTR style games, where thwe result of one adventuire is rarely determinant or relevant for the next adventure 8except as a way of hooking the characters into the next adventure)
Great ideas!