IR Thermometers are not a very good tool for measuring pinpoint temperatures. Think of them like a 1-pixel digital camera - you're getting more of an "average" temperature.
Moreover, note the digital "signature" (emissivity) of every material is different. Those IR thermometers are calibrated for measuring soft surfaces like wood and drywall. The temperatures measured of metal and glass will be extremely inaccurate because you cannot adjust the emissivitiy rating of most IR guns.
If you really want to measure the temp with an IR gun, you'll need to step up to an Infared Camera like a FLIR which allows you to select a lower emissivity rating for reflective surfaces like metal, but even then temperature readings will still be inaccurate unless you invest in an extremely highend FLIR with a high resolution camera
