Speed guns are very hard to use for clocking model airplane speed. We tried several types at my club for clocking the different classes of planes used in our racing class. They only way we could get them to work was to stand in the center of the runway with the plane flying towards us. There isn't a lot of metal in a model so we kept getting different readings or none at all. If you go into the extream speed prop plane forum they may know what guns work best but so far I haven't seen any one of the cheap guns that worked very well. Hope you have better luck then we did.