Raycast to follow an object on one axis, and another on a different axis

I'm not currently at my work station so I can't test this right away, but that certainly looks like something I can work with.

In order to do a couple of other interesting things, the point I have the ray aimed at is a sphere with the mesh renderer turned off (the renderer only continues to exist for visual debugging now and then). The sphere is a child of an empty game object that follows the mouse position (it also has a plane with some scripts for crosshairs and the like).

Do you foresee any major issues when applying your solution to this situation?

Thanks for your time so far. =)

As long as the aiming point is in 3D space in front of the camera, the code should work. Let me explain conceptually. Imagine there is a plane running through the gun with the plane normal facing up. For example, if the gun up was aligned with the world up, then this would be the XZ plane. Now project the aiming point on plane plane. Next a distance is measured from the gun to the projected point on the plane, and a vector is created from the plane up to the aiming point. Finally we construct a new aiming point that walks out the transform.forward the measured distance, then uses the vector to move up. So the final point will be directly in front of the gun but at right height.

Hello again. Turns out it's a public holiday today for some local show, but I did run a small test to try out your solution.

Here's the code.

gunMuzzleObj is just an empty game object representing the muzzle position of the gun, as one would expect.
bulletTarget is a sphere for aiming, in the same sort of setup I have for the crosshair.

 var pt : Vector3 =           ProjectPointOnPlane(gunMuzzleObj.transform.up, gunMuzzleObj.transform.position, bulletTarget.transform.position);
 var fwdDist : float =        (pt - gunMuzzleObj.transform.position).magnitude;
 var up : float =             (bulletTarget.transform.position - pt).magnitude;
 var adjustedPt : Vector3 =   gunMuzzleObj.transform.forward * (fwdDist + up);
 var adjustedDir : Vector3 =  adjustedPt - gunMuzzleObj.transform.position;
 
 function ProjectPointOnPlane(planeNormal : Vector3 , planePoint : Vector3 , point : Vector3 ) : Vector3
 {
     planeNormal.Normalize();
     var distance = -Vector3.Dot(planeNormal.normalized, (point - planePoint));
     return point + planeNormal * distance;
 }

The results seem kind of similar to what I've seen with my own attempts, with it seeming mostly accurate until you start to move around.

This is how it looks just aiming at a pillar/barrel thing from the starting position:

And this is how it looks as I move around and aim at different spots:

It seem to try it's hardest to match the target's position vertically (relative to the screenshots), but often falls short. Horizontally, it couldn't care less about.

My fault. I had the wrong type on 'up' in my code fragment (now fixed) and confused you. Your line 3 should be:

  var up : Vector3 = bulletTarget.transform.position - pt;

...and then line 4 would be:

  var adjustedPt : Vector3 = gunMuzzleObj.transform.forward * fwdDist + up;

'up' is the vector from the point on the plane to the target point.

Ah, there we go, that works much better. I seem to have gone the polar opposite than I should have when fixing that third line, woops. I should have thought to try both.
It aims in the right direction now and doesn't swing wildly about like my own poor attempts, but it seems to be aiming at a point further down and in front of the target than it should be. I'm sorry to keep bugging you, but I feel a little out of my depth here. Looking over the code, based on what I know (and have now learned) I don't see what's causing this.

Here's a couple of screens of what I mean:

In the 2nd and 3rd screen, I have it set so that I can toggle the target position between two spots (high or low) without moving the cursor itself.