QUOTE (ArkonC @ May 20 2008, 07:57 AM)

Actually, you need to cast it at force 5, beating the OR of 4 and 1 for actually hitting, needing a DP of 15 on average...
Not true. A spell against an object is a success test, with the OR being the threshold. You only need to get as many hits as the threshold to succeed.
"Hits represent a measure of achievement on a test. In order to succeed completely on a Success Test, you must meet or exceed a gamemaster-determined threshold with your hits. The higher the threshold, the more difficult an action is. The standard threshold is 1 (so only 1 hit is necessary to succeed), though other tests may have a threshold as high as 4" (SR4, 56).
QUOTE (Moon-Hawk @ May 20 2008, 08:11 AM)

I don't know. SR magic has always been pretty clear that you can't target a part of a thing, just the whole thing, which would imply that no, you can't blow a small hole in a windmill, you have to inflict abstract damage on the entire thing with a direct combat spell. Which gets into all sorts of awful confusion, is a door a thing, or a part of a thing (building)? etc etc. It's a headache, and I have no good answers.
You can target individual components, at least of vehicles.
"Called shots against vehicles follow the same rules as for Called Shots, p. 149. A third option, however, is available to the attacker if the called shot succeeds. The attacker can choose to target and destroy any specific component of the vehicle: window, sensor, tire, etc. The gamemaster determines the exact effect of this called shot, based on the DV inflicted. In most cases, the component will simply be destroyed. Shot-out tires inflict a –2 dice pool modifier per flat tire to Vehicle Tests" (SR4, 162).
In any event, I did some testing to see which type of spell really is better against a vehicle. My testing was limited, and in no way is statistically significant, but still it gives you something to consider.
I used my group's magician as the caster, who has 5 Magic and 5 Spellcasting, and no other modifiers, for 10 dice pool. I pitted him against a Bulldog van with a run-of-the-mill driver who had Reaction 3. He used Powerbolt and Acid Stream as the test spells. Neither character used Edge, to keep it simple. Against Acid Stream, the van driver just did the normal ranged defense; he did not use Evasion (full defense).
With Powerbolt, I first started him out using Force 5, but it quickly was obvious that he'd need to cast *a lot* of times to destroy the van. In the two tests I ran at Force 5, the results were as follows:
1. 20 castings to destroy the van; he took 3 Stun damage from drain.
2. 10 castings to destroy the van; he took 8 Stun damage from drain.
I stopped after those 2 tests, since even 10 castings is ridiculous. After that, I overcast at Force 9 to take down the van faster.
1. 4 castings to destroy the van; he took 6 Physical damage from drain.
2. 4 castings to destroy the van; he took 9 Physical damage from drain.
3. 4 castings to inflict 11 damage to the van, which was not destroyed; he took 11 Physical damage from the drain and was knocked out.
4. 2 castings to destroy the van; he took 3 Physical damage from drain.
5. 3 castings to destroy the van; he took 4 Physical damage from drain.
With Acid Stream, I used Force 7, which is was good enough to damage the van reliably, due to needing to inflict 9 DV or more. I didn't use Force 9 since the drain DV was already pretty bad (6).
1. 6 castings to destroy the van; he took 9 Physical damage from drain.
2. 3 castings to inflict 9 damage to the van, which was not destroyed; he took 11 Physical damage from the drain and was knocked out.
3. 4 castings to destroy the van; he took 9 Physical damage from drain.
4. 4 castings to inflict 3 damage to the van, which was not destroyed; he took 11 Physical damage from the drain and was knocked out.
5. 3 castings to inflict 9 damage to the van, which was not destroyed; he took 10 Physical damage from the drain and was knocked out.
Interestingly, the Powerbolt was more effective on the whole, despite needing to beat OR 4. I could try again with Acid Stream at Force 9 just to see, but I have a feeling that the magician would just knock himself out even faster.
If the driver had better Reaction, used Evasion, or both, then Acid Stream would be even less effective.