Third Person Shooter Blues

Long story short, I want to shoot a target. I have a reticle. I want a reticle that, as long as something is within the reticle, is considered targetted.

**How do I make a reticle that, as long as it is overlapping a valid target, flags that target?**

Doing a line trace doesn't work, because what I need for tracing is a cylinder or sphere trace And neither of those work because they seem to run into significant enough performance issues that I can't use them properly.