It seems to me that, by trying to make things better, you've made them worse.
What do we have in the RAW?
On the one hand, someone throws a PoF. He needs nothing for that, this works everywhere, is a sure hit, and needs to bypass MR
On the other hand, we have aimed stones. You need one (easy, but not always possible), which needs to hit and bypass MR.
Problem is, you require people to throw big rocks, which in turn, due to size, give aiming bonuses, making the "to hit" part easier.
What I'd do:
Stick to the RAW. It works.
Bigger rocks need Size magnitude. Also, bigger objects move slower. So a bigger rock won't do more damage (it goes more slowly) but will be easier to aim. You have a tradeof here. This should lower your Catapult of Vilano's damage by 5 or 10
If people want to do a "real" catapult (balistic trajectory) instead of a slingshot/firearm thingie, in order to use the fall to do more damage, sure! Increase damage by 05 by size magnitude of the rock, but then, don't forget that these are quite imprecise and difficult to aim.
If you really want to have them throw big rocks, I'd really go for something like JL's idea: These rocks are big, but they also more slowy, so this cancels the size bonus to aiming.
You must either cut the damage increase (my number 2 answer) or the bonus to aiming (JL). If you keep both, you're toast.
For what it's worth, your player also went to great lenghts for this, lenghts you in part forced him to go to. I'd let him enjoy at least some of his hard work without ressorting to what are, IMO, killer DM answers such as the bags exploding, the aegis magically cancelling his enchantment (Unless that's your HR on Aegises, of course, but then, you should warn him beforehand)...
I've seen it done that way in Novus Mane. Trust me, this is a bad idea.
Except, I think it's RAW. At least RAW is highly suggestive of doing this when it discusses aimed spells.
If you're treating it as an attack total, you have to be comparing it to a defense total. Once you have compared attack and defense totals, everything is derived based on the formulas, nothing is rolled again.
My question is why do you think this is a bad idea? I think I can guess why you think that way.
Err... bigger rocks do cause more damage. Otherwise treuchets would not be better than ballistae at deetroying city walls, and it is well nderstood already that the trebuchet is a better weapon at that. Idem fr faster and bigger trebuchets, if you can get them to work.
Yeah, I think we're mixing guidelines up and realizing abnormal results.
The guideline in Houses of Hermes is pretty explicit. It is throwing a rock with enough force to cause +X damage. I'm pretty confident that it is talking about sling stones, because the guideline was created to allow Invisible Sling of Vilano to be based on some guideline. The guideline even says stone projectile which is even more evocative of a sling bullet. The range increment is also consistent with the Sling in the Missile Weapon Table on page 177 of the main rule book.
Going back to size bonuses to aiming, you have to remember that it was intended for use with the existing guidelines. Moving a large boulder over a group is a ReTe Base 3, +1 stone effect. It also doesn't mention anything about speed, but I'm pretty sure we're not talking about ballistic or trebuchet speeds. I'm thinking something that takes a round or two of concentration to maneuver the boulder into position and then drop it on the unwitting foes, it moves as fast as a person can move. That definitely deserves a bonus to aim. It also suggests that there wouldn't be a bonus to evade, but I'd certainly allow dodge and cover to have an effect on the overall defense and soak.
In short,I don't think you can move boulders at ballistic speeds under existing guidelines. I do think you can extrapolate the guidelines in HoH:S to increase damage, but leave the projectile size as a sling bullet and increase damage as a function of an increased base (speed). I do not believe this suggests flinging large boulders about, with their incumbent aiming bonuses. If your troupe desires making large boulders move with ballistic speeds, I would suggest some alternative guidelines, and perhaps also base it more closely to what the Trebuchet does, as described in Lords of Men. Of course, the advantage for a magic lobbing +25 damage boulders like a Trebuchet is that the magus doesn't need the 2 minute reload time.
It think there needs to be a trade-off. Small rock can be given more momentum. Bigger rocks will cause more damage, by their very nature. However, they are slower and thus it should be harder to hit with them (because the target will simply see them coming and have more time to try to move aside).
Really big rocks cause huge damage and are actually area-effect.
Top me, this means that a rock-throwing spell can be boosted to do one of the following things:
Increase the speed of the rock, which increases it aiming bonus by 3 per magnitude.
Increase the size of the rock, which increases damage by 5 per magnitude but reduces the aiming bonus by 1.
Affect an area by using many rocks, thereby causing damage to everyone in the area.
The first two options are those that are being discussed here. I'll leave the third one aside for now.
So you can increase the speed to have better chance of a hit (aiming bonus), or increase the size for better damage. Or do a bit of both. Note that the bigger rock is more effective from a damage point of view, but speed offers better chances of a hit.
So if you have two magnitudes that can be used for boosting the spell, you could:
Use a regular-sized rock and give it a +6 aiming bonus. As the attack advantage carries over to damage, that potentially increases damage by 6 as well.
Use a much bigger rock and thus have +10 to damage, but with fewer chances of a hit because you have -2 to aiming. This reduces aiming carries over, still leaving the potential damage increased by 8.
You can use a slightly larger rock and increase its speed, for a total of +2 aiming bonus and +5 damage. That puts you mid-point at a potential damage increase of 7.
EDIT: As a final note, I would say that this kind of spell cannot be said to increase accuracy. Because accuracy would mean controlling the rock to its destination, which means to could be blocked by magic resistance. Thus, the best that can be done is to increase speed.
Something to remember, too.
BoaF can be used anywhere. Boulders? You can't throw them indoors. Likewise, you can't throw them at someone who's near a shield grog or sodales.
Nope. Attack Advantage for Aimed Spells do not carry over into extra damage. This was brought up here and discussed. IIRC, David Chart himself confirmed it is just a "to hit" roll. I remember that very much precisely because of Novus Mane.
Here: Building Finesse in to spells/devices - #39 by David_Chart
Of course, but we're talking of thrown rocks.
If bigger rocks are thrown, on a flat trajectory, with less force than smaller rocks, we can, at least for game balance purposes, assume that the 2 cancel out each other: You gain +5 because of size, you lose 5 because of lack of strength.
If going ballistic, so that the rock's fall will increase damage, he'll lose in precision, thus cancelling his aiming bonuses.
That really should be in the errata, because I don't get that impression from reading it. The moment you compare attack and defense totals, it begins smelling like combat.
Also, this could actually make aimed spells even weaker, when one includes soak bonuses for armor and shields.
For what it's worth, you're on the opposite side of your position from a few years ago.
Which just adds further strength my point it's entirely unclear, despite David Chart's quotation The Fixer provided the link to.
But a poor magus who takes Invisible Sling of Vilano as his only offensive spell and manages to get off a successful attack against a mundane in chainmail is unlucky indeed. Let's say he succeeds in the attack, and go by RAW as clarified by David Chart. Now opponents roll for damage. On average the magus is going to do +11, and the mundane in chainmail is going to be able to soak 15 just by dint of his armor, not counting stamina or anything else. Unless one thinks armor shouldn't apply to this spell, but I have a hard time justifying that given that this is essentially a mundane attack. This applies to anyone Leather Scale and above, on average Invisible Sling of Vilano will not do any damage, and the only way to deal damage is with exploders or a botch on the defensive side.
I know, this suggests the need for another spell, like Wielding the Invisible Sling or Pilum of Fire which can be presumed to deal damage directly, bypassing armor. But Wielding the Invisible Sling has the armor issue, too. Some troupes play Pilum can be soaked by armor for the first attack, but not subsequent Pila of Fire...
I have not reviewed it recently, but we do have had aimed spells IMS (manx saga), and we have been using carry over to damage. It was not thought to be unbalancing.
Well, as I suggested with my example, I think it might even be balancing. If someone's going to invest a lot of experience points in finesse, there should be a benefit that carries through.
I think there's something else to consider about whether or not to roll for damage on the aimed spells. The magus who relies upon finesse based attacks now has three opportunities for his spell to fail. The spell roll might botch (this can be mitigated by mastery, but adverse auras make this harder), his finesse roll can be botched (this can be negated by Cautions with Finesse) and finally his damage roll can botch, and it is explicitly mentioned on page 181. I'm ok with two chances to fail. Three just starts to seem punitive...
I follow David Chart's reading. And think it's the wise choice as it is simpler and more consistent to run (spell X produces X results; simple, easy), and so that Finesse uber specialization doesn't unduly increase damage.
It allows the character to master the spell for fast-casting and cast a handful of such spells in the same round. That's, ahem, enough of an advantage...
(I hate multiple-casting with the fiery intensity of a thousand exploding suns... and fast-cast is worse...)
Because regular mastery is not enough to break the system with any combat spell, eh? The only real result here is that you are making penetration more desirable than finesse as a hermetic ability. Brute force beats skill. I do that all the time with my characters, but I do not like it conceptually.
Not sure how finesse uber specialization is any worse than thrown weapon uber specialization or Major Magical Focus (damage) is an uber specialization.
Multiple casting needs to be treated as quick and precise casting are, IMO; take X number of times to add X number of copies. When dealing with aimed attacks, this is also limited by the ammunition available AND it has a penalty to hit. Three copies to three diferent people, each is at -3 to hit, for each one. Even three copies at the same person, each one is a -1 to hit for each one.
Applying multiple casting to my previous example of the well armored opponent won't affect the damage result, on average. At best you are hoping for luck. The fire damage specialist doesn't have that concern, doesn't have any of those concerns, and can likely achieve penetration of his effect pretty easily if the individual has MR. If I have a bag of 20 sling stones and Invisible Sling of Vilano, with a Mastery Ability score of 3, and multiple casting taken once and everything as per RAW, it's unlikely that any of the 4 sling stones will do any damage, even assuming they hit. Mastery, for aimed spells, is primarily useful to mitigate against a botch.
Fast-casting, IMO, should only be allowed as a defensive measure. I think that is the original and intended purpose of fast casting by reading the section very carefully. I really, really hate (that might be too light a word) the idea that you can beat your own initiative and your opponent's, and somehow manage to attack before the opponent gets his attack off, by a roll that gets a bonus of Finesse added to it, unlike the original Initiative roll did. Fast-casting should be for mitigating an attack, or, acceptance of mutually assured destruction. Alpha and Beta are fighting, Alpha has initiative, Beta loses it. Alpha swings with a sword, but Beta fast casts a spell. The appropriate and reasonable fast cast spells are things that mitigate the result of the sword swing (Wizard Leap, something that turns the Sword into Butterflies, or something), not a Ball of Abysmal Fire Beta tries to get off before getting hit with a sword. It very well may be that Beta has no suitable response, but to avoid getting hit. Going out in a blaze of glory is entirely reasonable.
For example, I have a maga in Oppidum of Héviz who mastered Mighty Torrent of Water for fast casting. In that saga fast casting is only for defense or the mutually assured destruction. I can fast cast it against PoF and BoAF, or other fire related magics, but if I try and use it as an attack and fast cast it, I'm still getting hit with whatever was coming.
: shrug : High Finesse allows a character to fast-cast defensively, as well as to aim spells and - by RAW - to fast-cast offensively. And it has occasional other uses. I think that's a very useful ability, generally more useful then Penetration that allows the character to add multiples of it using arcane and sympathetic connections and that's about it. (Yes, he also adds it once per spell, plus once more with Mastery - seriously ? You're excited about adding +4, day, to your spell's penetration total ?)
My changes requiring minimum size only make Finesse less applicable, as one could add large size anyway and I'm just requiring it, thus making aimed spells less flexible and potent.
Uber thrown weapon specialization will rarely benefit from wizardly shticks like Fast-Casting. Major Magical Focus (damage) is silly, granted, but one silliness does not undo another....
The big problem here is that this is even allowed. Instead of casting one spell per round, you're casting several, multiplying your damage output. That's major.
One way to address multiple-casting is as a limited from of Flexible Magic, allowing you to change the Target of your spell from Individual to Group (at a +10 increase to the spell's level). This is at least consistent with the magic system, and also opens the door for other Flexible-Magic based special abilities. It practical terms it means you invest time in Mastering your spell instead of learning a similar, Group-target, one, which does not seem unreasonable.
Another way, once suggested by The Fixer, is to treat each multiple copy hitting the same opponent as an extra magnitude (+5) of damage. At least that keeps multiple casting from overwhelming individual opponents.
My favorite way is to nerf it totally. Make the mastery ability allow you to divide up your Casting Total. If you actually have enough Casting Total to cast the spell multiple times, it probably isn't that dangerous to let you do so...
I don't have a problem with a sling not affecting an armored knight. Or a scaled dragon. Except for luck.
This is only as long as you keep aimed spell damage down. If aimed spells do large damage - such as increasing the damage based on the HoHS guidelines, or using a "drop a rock" spell - then suddenly being able to aim better and fast-cast several in one round is nice.
That makes a lot of gaming sense, but I find it makes little in-world sense. My troupe discussed the matter and we settled on house-rules that basically limit offensive fast-casting but do not eliminate it as an option. In case anyone is interested, they are:
Amir’s Fast Cast Rules: By group agreement, our house rules for fast-casting are as follows:
Normal Spell Initiative is (die + Quickness). Fast-Cast Initiative adds +Finesse on top of that total (for the combat).
You have a pool of fast-cast spells you can cast each round in the combat. The size of this pool is determined by your Fast-Cast Initiative for this combat. It is 1 fast-fast spell at EF 5, 2 fast-cast spells at EF 15, 3 fast-cast spells at EF 25, and so on.
From the first time you cast a fast-cast spell in a round, you can no longer cast a normal spell in that round. From the first time you cast a normal spell in a round, you can no longer cast a fast-cast spell in that round.
From this point on the rules depend on whether the spell is Personal, i.e. affects only the magus and his gear, or non-Personal.
A magus may fast-cast a non-Personal spell on his Fast-Cast Initiative or his Normal Spell Initiative. He cannot fast-cast a non-Personal spell at any other point in the round. A non-Personal fast-cast spell is not an interrupt, and may provoke an interrupt Personal fast-cast spell.
A magus may fast-cast a Personal spell on his Fast-Cast Initiative or any lower initiative count. A Personal fast-cast spell is an “interrupt” action that, if successful, will prevent the interrupted spell or attack from affecting the magus and his gear. It does not provoke an interrupt (Personal) fast-cast spell.
A magus may continue to fast-cast until his fast-cast pool runs out (for this round), subject to the limitations above.
But someone with a high Thrown Weapon Score could damage a knight or a dragon with a sling, without luck, possibly. In a game about magic, magic should generally trump the mundane. Except in this instance, it doesn't. Because the RAW as clarified by David Chart says that the Attack for a spell is a yes/no check, and any overage is discarded, and then roll for attack. The guy with a sling gets his Thrown Weapon score + die + 1 From the sling vs the Defense total against the knight or the dragon. On average they will be fine, but on the outstanding success yes, there will be problems. My main issue is that by discounting the spell success roll, the finesse roll, and leaving it all to damage, is that the finesse player is actually penalized if he should roll well before the damage is done. That's kinda a bummer. You rolled 81 to hit. But you botched your damage roll. Sorry, dude! That doesn't happen in combat, and again in a game about Magi, magic should trump the mundane.
Alright, scale it up per the guidelines, nothing changes, and it is no better than PoF or BoAF for doing damage as a function of magnitude, and it also requires ammunition being available, and good finesse scores. Or revise the guidelines and make variants of the Crystal Dart and me a MuReTe specialist and ignore having to improve finesse, and focus on penetration, because then I don't have to worry about aiming and all three of my Crystal Darts or their variants will hit.
Of course it makes game sense. I'm not sure what you're going at by saying gaming sense and in-world. There are numerous spells that can be used to mitigate receiving the effect of the attack. Wizard's Leap, and you're out of the way.
This just nerfs fast casting, generally and makes spell mastery for fast casting even more important. By preventing a normal spell for being cast when someone has fast cast, or someone from fast casting when they have already cast a normal spell, you're just making combat focused players chose fast cast mastery for all of their offensive and defensive spells.
Ah, now I get your point. Good point. Good two points, actually.
Yes, the sling weapon can out-damage the sling spell. I can see where this doesn't fly in a game about wizards, certainly. More broadly, however, we're allowing high Finesse here to count as a weapon ability. All weapon abilities, with the right spells. I'm not sure I like that. Even if the focus of the game is wizards, I don't like that they can fight like a trained fighter. Leave the weapon-fighting to the mundanes, or the few martially skilled magi, and leave the wizards to do... wizardry stuff. Also, not allowing this means that wizards will seek more impressive spells rather than increasing their Finesse, which I like. I can see both sides as having good points here.
On the issue of rolling high hits in vain - yes, that sucks. Another very good point for using the attack advantage method.
Let me put it this way -
Take a high Finesse character, and let him gain fast-cast mastery in BoAF and a Catapult of Vilano. Under RAW, he can fast-cast lots of BoAF or lots of CoV, as he wishes, each round. That's very useful. With one Mastery point, 5 XP, a Season of Practice, he gets to cast multiple copies of the spell per round. That's very, very effective use of time.
Now, CoV has the disadvantages you noted. The character might not want to cast it if he has no suitable ammunition, or even simply if targeting opponents with low magic resistance (so penetration isn't an issue). In those circumstances where he does want to use the CoV, however, his high Finesse also aids his attack rolls. That's cool.
And his high Finesse means that he is more likely to fast-cast defensive spells, and to react in time to cast offensive or reactive spells in combat.
These are all major advantages. In my opinion, that's superior to accumulating +4 on the penetration of all your spells, plus a heftier bonus on that rare and half-mythical case where you obtain an arcane connection and sympathetic connections to your opponent. Not that I'm dissing Penetration - it can be a powerful Ability in those rare cases. But I think those cases, important as they are, are rarer.
So I think both Finesse and Penetration are useful Abilities, and I'm not too concerned if one is slightly more useful than the others.
Sure. But why can you fast-cast a spell defensively, but not offensively ? You can fast-cast a torrent of water to smother that incoming fireball... but not if you aim a little to the side, to hit the magus' shield grog instead ? Sorry, this kind of distinction between spells doesn't really work for me. From a gamist perspective, I can grok it and ignore that. But from a simulationist perspective, it still doesn't jibe.
Hope that clears up my point.
Well, the RAW already disallows casting both a normal and a fast-cast spell. Arguably. (There was a big discussion on that recently; I think this was the consensus.) So this isn't a change, just a clarification. And yes, this does mean that combat-focused players will choose fast cast mastery. Which leads to them focusing on Finesse, which leads to them looking up other ways of using Finesse, such as bypassing Magic Resistance by using aimed spells, which leads us to where we started this thread...
The house rule does indeed nerf fast casting in general, which makes fast-cast mastery even more important. Note it also means that all those Finesse-maximizing characters are actually worse off than under RAW. This was what my troupe wanted. I'm not particularly enamored of this house rule, it's rather clanky and weird - but so be it.
We hardly ever collect arcane connections. That might explain our POV difference too
Again - getting high bonuses from arcane and sympathetic connections is great and justifies investing in the Penetration Ability. It's a useful ability. It's just that Finesse is useful too - to be the fastest gun in the west, errg, the fastest spell slinger in the combat, and to do cast many, many fast-cast spells instead of one (plus a few other uses).
The only real result here is that you are making penetration more desirable than finesse as a hermetic ability. 
Brute force beats skill. I do that all the time with my characters, but I do not like it conceptually.