- Overview -
Using the law of Sines, this concept shows a accurate simple way to have missile-to-wall collision.
Using basic trigonometry, this is achievable.
Please use this concept in your map if you will. Please credit this system if you do use it.

- How it works -
A player controls a unit such as a Photon Cannon in a rectangular battlefield with 4 corner points.
When the unit attacks, 4 if/then/else actions, determine which side of the battlefield the angle of attack is within. (which wall the missile will contact)
The triggers will then create a virtual triangle to determine where the missile will go. 9 variables consisting of 3 sides, 3 points and 3 angles.
The first point is the location of the Cannon, the second is a corner of the battlefield, and the third is the impact location. (what we are solving for)
The first side is distance from the cannon to corner, the second and third are from cannon to impact and impact to corner. (unknown lengths)
The first angle is the angle from the cannon to the corner contrasted to 0 degrees, the second is from the corner to the cannon contrasted to 0 degrees, and the third unknown.
Using the law of Sines and knowing Angle Side Angle, we can find the missing variables.
Then the triggers will simply create a missile unit and send him to the third point for impact. Voila!

This method uses very little system resources and is 100% accurate.
I've uploaded a demonstration map to battle.net to see for yourself. [Trigonometry Cannon Concept, by bolshevik09]
Please try it out and leave feedback on this forum. Thank you!

You can view pictures here