This is the famous Exergen DX501 Infra Red Temperature Gun. This is the most accurate temperature gun you can buy, and is what most of the top drivers use to measure the temperature of their engines. This temp gun is the most accurate you can get, so if you want the best, this is it!
Note that like all Infra Red temperature guns that have an exposed tip, it is always a good idea to keep the end of the gun clean. If you get dirt or debris in the end of the gun it can affect the readings, so spray a little denatured alchohol on the end of the gun and blow it off with an air compressor. Also make sure the batteries are fresh, as dead batteries will also make a temperature gun measure incorrectly.
The DX501 temperature gun can also be re-programmed to display the temperature in Celcius instead of the default setting of Farenheit.
NOTE: If your temperature gun starts to read values that appear to be off, it probably means the battery is going flat. Please replace the battery, and the readings should be accurate again. Also make sure the tip is clean, and if it gets dirty, clean it with an alcohol cleaner like Denatured Alcohol and an air compressor.