Another factor to an accurate temperature reading with a temp gun is whether you are using it correctly. Sure a temp gun may work from 20 feet away, but you are not getting the actual reading where the red dot is. You are getting a reading of the ambient temperature of the air. Also, the temp gun will only give an accurate reading when taking the reading from a DARK surface and very close to the heat source. I've proven this to many people.

Just try this test with your temp gun:
Turn the furnace on in your house and let the ducting get nice and warm. Take a piece of black electrical tape and place it on the duct. A one inch piece will do. Now from about one inch away, take a reading on the duct itself, then take a reading off the black tape. You'll be surprise how different the readings are. Then take readings from further distances, the farther away, the less accurate the reading is.

So, most engine cases, the head and plug are silver in color and will not give a true temperature reading.