Re: Reverse Missiles / not an "attack"
 

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."

Re: Reverse Missiles / not an "attack"
 

GURPS ranged attacks are hitscan weapons*, and wherever you are when you make an attack, that's where you are when the projectile returns to you - there is no travel time involved. You need look no further than the penalties for Range to determine this - something that is twice as far away looks half as large, and lo, both situations (twice as far, or half as large) result in the same -2 to hit (you can repeat this for anything else; basically, Range penalties just treat the target as though it were 2 yards away but only as large as it appears... which is exactly how hitscan weapons work).

If you're wanting to get more simulationist than the default, you'll need to make some decisions. How does the spell determine the point of origin, and how is this point fixed in space? Maybe the projectile "remembers" where it came from, and the spell tells it to go back home. The origin point is obviously set relative to the planet (otherwise, with how quickly Earth hurtles through space, I suspect anything beyond point-blank would wildly miss the attacker when it returned), but what else could it be set relative to? A vehicle (so Reverse Missiles still works as-advertised on a speeding train)? A mount (so that horse archer isn't immune)? A shooter (so a teleporting mage isn't necessarily safe.... although I'd argue the projectile should still be subject to 1/2D and Max Range under such a simulationist handling)? What happens if there's something in the way of the projectile returning, that wasn't there when it initially departed? How you answer these questions may well modify how Reverse Missiles functions in your game.

*For those unfamiliar with the term, this is the way projectile weapons work in many - mostly older - video games. When you attack, rather than your weapon producing a projectile that must collide with your target's hitbox to have an effect, the game simply checks to see if your aimpoint is in line with said hitbox when you attack - if it is, the target immediately takes damage. There are some games where there's a visual effect that's slower than the instantaneous damage effect, which can have humorous results - such as a foe falling dead, then being struck by the fireball that killed it (or an arrow fired at an erratically-moving target gaining homing capabilities).

Re: Reverse Missiles / not an "attack"
 

You are allowed to dodge the returning attack.

Re: Reverse Missiles / not an "attack"
 

However, that requires that you can see it coming, or otherwise be aware of it. This is fine for large, slow missiles, but not for bullets.

Re: Reverse Missiles / not an "attack"
 

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.

Re: Reverse Missiles / not an "attack"
 

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.

Re: Reverse Missiles / not an "attack"
 

Or worse case scenario; the original 'archer' scores a critical hit with his 'arrow', he gets no defense since the incoming Reversed attack is still a critical hit.

Re: Reverse Missiles / not an "attack"
 

However with a homing or multiple attack projectile Missile Shield gets to a comedy level. A bullet with such properties fired on a called shot head would FREX somewhat resemble an Ioun Stone.

Orbit and Orbit the head, never to strike...

Re: Reverse Missiles / not an "attack"
 

A Critical Success should never be detrimental to the character rolling it. In this case, at worst I’d have the reversed missile wind up missing the attacker. I’d honestly be tempted to have something like a 1/6 chance of it bypassing the shield and perhaps a 1/6 or 2/6 chance of getting deflected into another target; in either case, there would be no roll on the Critical Hit Table.

Re: Reverse Missiles / not an "attack"
 

No, it isn't. Actually I'd argue that a critical hit would at the least be a guaranteed miss by the Reverse Missile spell because critical hits mean you got lucky.