This depends directly on your other query, regarding zoom effect.

Ideally, you can set your limit to the distance upto which your view doesn't get inverted.

Well, if you're trying to figure out the maximum distance your weapon can fire efficiently then it is the value at which your FOV changes to an upside down world.

That's how I meant that this is related to your other query.

@Paradigm Shifter: No, it's not backward, I'm trying to cast a ray from the cross hair to the forward direction (limited to certain range)

Take a look at what I'm doing:

D3DXVECTOR3 ScreenToWorld(HWND Hw,float X,float Y,const D3DXMATRIX &View,const D3DXMATRIX &Proj)
{
     RECT R; GetClientRect(Hw,&R);
     D3DVIEWPORT9 Vp;
     Vp.X=R.left;
     Vp.Y=R.top;
     Vp.Width=R.right-R.left;
     Vp.Height=R.bottom-R.top;
     Vp.MinZ=0;
     Vp.MaxZ=1;
     D3DXVECTOR3 V(X,Y,0);
     D3DXMATRIX World; D3DXMatrixIdentity(&World);
     D3DXVec3Unproject(&V,&V,&Vp,&Proj,&View,&World);
     return V;
}

If you unproject both (X, Y, 0.0f) and (X, Y, 1.0f) you can use the difference between those two positions to find the direction you need.

If you normalize that direction vector and then scale it by the distance parameter you'll have your destination point. Just make sure you do the subtraction in the correct order or you'll be shooting backwards.

The cross hair isn't the origin... it's the target. The gun muzzle (or the camera) is the origin...

So, that means I was not working on the correct way, I changed the code to:

D3DXVECTOR3 rayFrom = camera->GetPosition();
D3DXVECTOR3 rayTo = ScreenToWorld(...);
Raycasting(rayFrom, rayTo);

Is that the correct way?

Let me know how I can set the distance (range the bullet can hit).