The Bushnell is accurate. It reads well at lower speeds. We use the Prospeed and Stalker pro and Stalker sport radar guns for Dynamic soaring.
Many have tried the Bushnell, for our uses it is not usable, but for models and things up close up to around 100mph they are OK (we went 313mph 2 weeks ago with a glider!).

Radar guns NEED the object to be Dead in-line, either coming At, or going away directly from the gun, and the readings will be accurate. As the angle derivates from in-line, the readings will be lower.