Reverse Missiles / not an "attack"

[Varyon]

Originally Posted by ravenfish

I am not concerned about air resistance bringing the projectile to a stop, but rather about the projectile being slowed enough that gravity has more time to pull it down before it reaches the target. A marksman firing at long distances can take this into account when aiming; I strongly suspect that the Reverse Missiles would have to make further adjustments when firing the missile back.

If what the spell does is reverse the projectile's velocity state, that could be an issue. But that's not what the spell does - it specifically sends the projectile back at the shooter, which may well require it to arc the shot or similar.

As to "hostile intent" (which I don't think is part of Reverse Missiles, but apparently is part of some spells), that doesn't necessarily require any sort of mind reading. In many settings - particularly various flavors of shounen anime - "hostile intent" is a detectable force/energy that is produced/released by anyone acting as such. It may be possible for someone to engage in hostile action without any hostile intent, but this is likely to be the exception rather than the rule (there's an antagonist in Rurouni Kenshin who has sufficient psychological damage as to not generate such). How to handle that - and if it's even an option - would ultimately be up to the GM.

[mlangsdorf]

Originally Posted by ravenfish

This is a tempting reading, but it runs into the problem that, due to air resistance slowing the projectile both coming and going, a strict reversal of its direction would almost certainly fall well short of a shooter firing from anything greater than point-blank range [EDIT: I assume, at least- can anyone who knows about ballistics analyze this?].

Assuming negligible aerodynamic lift from the projectile and a flat path to the target, a bullet fired from an aimed position needs to fall about 1.5m to hit the ground. Gravity acceleration is 10 m/s^2 or so, and the equation to calculate accelerating movement from a standing start is D = 1/2at^2, or solving for t, t = sqrt(2D/a). A is 10, D is 1.5, t is 0.54 s or 0.44 s when firing from the hip, 1 m above the ground.

Typical .45 ACP rounds have a muzzle velocity around 300 m/s (+/- 10%). So if you fire a .45 ACP at a mage with Reverse Missiles who is 90 yards away, it's possible the round would hit the ground instead of reversing into you. But it's unlikely if target is much closer, and if the target is farther away, the bullet is going to be fired at an upwards angle out of the gun so that the bullet drop lets it hit the target, and that complicates the analysis some (or a lot, depending on the range).

Guns that fire faster bullets are going to have relatively longer ranges - a supersonic .308 round covers 350+ meters in 0.5 seconds, for instance, and the optimum range is going to be closer to 200 yards.

For simplicity, I'd pretty much ignore the possibility of a projectile reserved by Reverse Missiles hitting the ground unless the target is beyond half the maximum range of the attacking weapon. If I was feeling really generous, I might make that a quarter the range if the attacker is aiming at the target's feet (or a prone target, etc) and the projectile starts the reversal much closer to the ground.

[ravenfish]

Originally Posted by mlangsdorf

...

Thank you- that's the sort of analysis I was hoping someone here could do. So, for fire at reasonably close range at least, a "dumb" Reverse Missiles would be perfectly functional, but the spell probably has some active targeting component if it still works at great range.

EDIT: For my "Mages of the Enlightenment" setting, which at least makes a pretense of spells functioning based on the simple laws of the universe rather than the caster's intent, and at any rate there is a fairly hard rule that magic has no more intelligence than the caster put into it, I may decide to houserule that the spell is the "dumb" version and steal your ruling about half maximum range- a simplification, as you say, but playable.

[Fred Brackin]

Originally Posted by ravenfish

EDIT: For my "Mages of the Enlightenment" setting, which at least makes a pretense of spells functioning based on the simple laws of the universe rather than the caster's intent,

Reverse Missiles doesn't divine intent anyway. It responds to _actions_.

So, assume you have a magical gun that heals beings struck by its' projectiles. Those healing bullets would still get reversed even though they are fired with _helpful_ intent.

[RGTraynor]

Originally Posted by ravenfish

This is a tempting reading, but it runs into the problem that, due to air resistance slowing the projectile both coming and going, a strict reversal of its direction would almost certainly fall well short of a shooter firing from anything greater than point-blank range. Unfortunately, this may ultimately be one of those spells that "does exactly what it says on the tin by inexplicable magic"- it works out where the attack came from, and uses the incoming missile for counterbattery fire.

My take is that we need to take into account what the spell's supposed to do, which is to attack the attacker with his own missile. If air resistance was to be a factor, one would expect that the spell would have language along the lines of "Double the range for the purposes of the attacker's 'to hit' roll, as well as assessing any resulting damage."

[Anthony]

Originally Posted by ravenfish

This is a tempting reading, but it runs into the problem that, due to air resistance slowing the projectile both coming and going, a strict reversal of its direction would almost certainly fall well short of a shooter firing from anything greater than point-blank range [EDIT: I assume, at least- can anyone who knows about ballistics analyze this?].

In a vacuum, reversing path will work unless the attacker was moving (in which case it will wind up returning to the point the projectile was fired from). In atmosphere, it's not going to work correctly for long range fire, but it's probably safely ignorable at normal combat ranges.

[Plane]

I think it the case of shrapnel it would revese back to center of AE rather than where missle was launched, same with a grenade.

[Curmudgeon]

Originally Posted by ravenfish

This is a tempting reading, but it runs into the problem that, due to air resistance slowing the projectile both coming and going, a strict reversal of its direction would almost certainly fall well short of a shooter firing from anything greater than point-blank range [EDIT: I assume, at least- can anyone who knows about ballistics analyze this?]. Unfortunately, this may ultimately be one of those spells that "does exactly what it says on the tin by inexplicable magic"- it works out where the attack came from, and uses the incoming missile for counterbattery fire.

The question is complicated by the fact that some Protection and Warning spells do throw in a mind-reading effect as a free bonus- Watchdog, for example, explicitly checks whether an intruder has "hostile intent" (which raises a host of problems itself- does it fail to warn you when mindless zombies or golems close in to attack, to say nothing of question of using magic or Mind Block to mask hostility).

EDIT: As to the particular question the OP asks, I would rule that "attack" in no way implies "particular hostile intention", and an incoming projectile is returned to its source with no regard for the reason the source lobbed it. If there were any doubt, the Kromm quote mentioned up thread (saying that even shrapnel is deflected back to the site of its source explosion) would force us to conclude that the "attacker" doesn't need to have any intent at all, so hostile intent is clearly not a requirement.

Meh. It's magic, not science. If you need a scientific reason for it to do what it does, here's one that couples magic with science and doesn't require any special aiming effect. The missile itself turns end for end in both the x and y axis so that it's pointy end is facing the shooter (more or less), everything thereafter is simply the missile remembering where it's been (essentially Law of Contagion), one point at a time. As part of its remembering, it remembers the velocity it had at that point and resumes it. In effect, the effect of air resistance on the return path is negligible because the missile is accelerating back to its original muzzle velocity (or equivalent) as it reverses along its course and arrives with full damage potential (i.e. 1/2 D range damage, not Max range damage).

[malloyd]

Originally Posted by Curmudgeon

Meh. It's magic, not science.

Yeah, that. Note that any kind of physical reversal process runs up against the problem of what happens if the shooter moved, and the more serious one that it would almost always fail to threaten the shooter even if he has not. The missile should hit his weapon, or smoothly return to his quiver, or pass through the point in space where the end of his sling was when he released the string....

The downside of the magic being able to hit the shooter anyway is that logic would suggest it should be fairly trivial to design a variant spell that uses a similar process to propel something the mage tosses randomly into the air so that it infallibly hits the eye slits of anyone he designates as the "shooter", possibly at hypersonic speeds at ranges over the horizon, given that the Reverse spell seems to work on such projectiles. Now you need a second justification for why magic does something that doesn't make sense to prevent that, which is likely to require a third and a fourth and...

It may be better to just call it from the beginning, it's magic, it doesn't make sense, and stop wasting time trying to figure it out or develop exploits. The downside there is some exploits are pretty cool, and it's a shame to lose them.

There really is no good way to balance all of those in a way that doesn't involve case by case arbitrary rulings. Mind you if magic is sapient, which a lot of evidence seems to support, the spell spirits or whatever might well [change their minds] from casting to casting and alter what exploits do or don't work from day to day.

[ravenfish]

Originally Posted by Curmudgeon

everything thereafter is simply the missile remembering where it's been (essentially Law of Contagion), one point at a time.

Okay, that I'll buy. It still doesn't cover the missile's ability to track a moving shooter, but, as you say, it's magic.