Most of the cheaper ones have emistivity set to approx 100 this means they will only be accurate when pointed at a matt black surface, more expensive ones have adjustable emistivity meaning they can be set to be accurate on all materials but you need to know the emistivity (reflectance) setting of the surface you are measuring.
Do not believe the specs quoted by the manufacturers these are under non real conditions ie being pointed into a black body sources which are used for calibrating them and at a certain distance from the source.
In the real world they are good for comparison readings but not great for very accurate readings.
If you want a more accurate reading measure the temperature of what you want with a thermocouple&Display then adjust the emisitivity (presuming you have a more expensive one) to make the IR Gun read the same you can then use that emisitivity setting each time to get a more accurate reading (also make a note of the distance as that has an effect as well.
