As has been mentioned, we are looking for abnormal temperatures rather than absolute temperatures. I believe any of the guns will do a good job for most of our needs.
As has been mentioned, the reflective characteristic of the surface can significantly change the reading. More expensive guns will have adjustments for emissivity. That sounds good, but in the real world, you would need a calibration system. In our belt test lab (when I worked at Gates) we used a calibrated heat chamber with several samples in the chamber. Because of the chamber, they were all heated to the same temperature. We would then determine the emissivity level setting for each type of surface (belts, sheave, painted bearings, etc.). We needed the absolute temperature for our data logging. However, that was in a LAB. In the real world, we don’t need to know the temperature exactly.
As an example of emissivity, I suspect JohnZ, saw temperature variations for different items in the room. Indeed, they all should have been about the same temperature, unless selected items were getting heated by exposure to the sun through a window.
Bottom line, the cheaper guns should be fine for what we use them for.
Having said that, I firmly believe that trying to tie tire temperature as some sort of measure of tire pressure is bordering on a dangerous process. Obviously, a severely under-inflated tire will be hot. However, I strongly suspect that you can not sort out a 15% under-inflated tire from the others because of the variation that occurs from exposure to the sun, difference from wind-loading, inside vs outside dual, etc. The same goes for “thumping”. Temperature and thumping might tell you of a severe condition, but not warn you of a long-term damage causing problem. There is simply no substitute for measuring the tire pressure with a gauge or tire pressure monitor.
Now to unique uses of a temperature gun. My wife has rheumatoid arthritis. One of the symptoms is elevated temperature at the affected joints. When she has symptoms of a flair-up, we get the gun out to see if it is truly a flair-up of RA or her osteoarthritis (the common ailment that many of us suffer from if we have a few years on the old body). It makes a huge difference, since the two ailments are treated quite differently. The RA requires increasing dosage of some really nasty medications to prevent accelerated damage to the joints, as opposed to generic pain killers for the osteoarthritis.
I often mention this unique application in my seminars and Pat always gives me a “crusty” if she hears me.