RE: Radar guns
All radars lose accuracy when the object being targeted is at an angle in relationship to the receiver. If you understand how they work--sending out a pulse, and reading the time it takes for that pulse to "bounce" back, then you'd have not problem understanding that they are most accurate when the object is coming straight at them--along the same horizontal plane. Start raising the horizontal plane of the plane, and the gun becomes inaccurate.
My brother has one--I believe the same one you are talking about, and it works good. We have a blast with it. Kind of fun to at least get some idea of how fast the model is traveling!