Stirges (clarification)

[ThirdWizard]

I want a spell with a Tarrasque as a material component.

Wizard: I move next to the Tarrasque and cast my specialized detect magic with a Tarrasque as a material component!
DM: You arn't really in possession of the Material Component. You'll have to do more than touch it... make a grapple check and pin it.
Wizard: Um... urk...
DM: The Tarrasque allows you to grapple, and doesn't fight back!
Wizard: Awesome, next round I'll have him pinned!
DM: The Tarrasque eats you on his turn.
Wizard: I always wanted to play a Dwarven Barbarian anyway...

 

[RigaMortus]

So, some actual rules here?

Ranged attacks into a grapple strike a combatant determined randomly.

That's all it says. No method to determine percentages or anything.

I think it is fairly common sense how you would determine things randomly.

 

[Brother MacLaren]

I think it is fairly common sense how you would determine things randomly.

Really? Common sense? How so? It clearly shouldn't be a 50/50 chance in all cases.
If the tyrannosaurus is grappling the halfling (and about to swallow), what is the chance of hitting the halfling if you shoot at the dinosaur? What if you swing at it?
Now, you could account for size this way: every size difference is a multiplier of 4.
A human grappling a gnome has 4 times the size (in terms of frontal surface area). So there's a 1-in-5 chance of hitting the gnome and a 4-in-5 chance of hitting the human. Or should that be a factor of 2 for each size step to make the math easier when large differences are involved?

 

[RigaMortus]

Really? Common sense? How so? It clearly shouldn't be a 50/50 chance in all cases.

Nobody ever said it would be 50/50 in all cases. You just proved my point. It clearly shouldn't be 50/50 in all cases. That is common sense. If 3 people are involved in a grapple and someone fires into that grapple, that is a 1 in 3 chance you'd hit the intended target. Or, in D&D terms, you have a 1d3 chance in hitting them. Common sense...

If the tyrannosaurus is grappling the halfling (and about to swallow), what is the chance of hitting the halfling if you shoot at the dinosaur? What if you swing at it?

By the RAW? 50% or 1d2 (pick your method and roll).

Now, you could account for size this way: every size difference is a multiplier of 4.
A human grappling a gnome has 4 times the size (in terms of frontal surface area). So there's a 1-in-5 chance of hitting the gnome and a 4-in-5 chance of hitting the human. Or should that be a factor of 2 for each size step to make the math easier when large differences are involved?

That would be a house rule. Use it at your own discretion.