Well... these infrared temp guns became popular first with the RC car guys. Cylinder head temp offers a good way of tuning their air-cooled glow engines without the benefit of being able to get an "RPM" reading. RC car engines don't have the blast of cooling air from a big ol' prop skewing the results, so the cyl head temp taken directly atop the glow plug is fairly accurate. Plus glow engines tend to benefit from the cooling affect of a richer mixture much more apparently, so tuning by CHT is actually a fairly good method when RPM reading is not practical.

However, to get a good reading, you will have to hold the gun less than 1" above the head to get a good reading. A good temp gun will give a dispersion ratio, something like 4:1, which means the the target becomes 1" wide if you move the gun 4" away. 1" away from the plug on a small engine can mean a significant difference in temp.

Now... their use on airplane gasoline engines is a whole 'nother ball game. A lot of the cyl head temp quotations, even the ones published in magazine reviews, are clearly skewed by the prop blast. Readings taken on a 50F ambient temp day is vastly different from a 90F day. So what good does it do to list CHT for an engine with different props when they are all less than 250F (as one recent mag article did)? Also, as long as you are not pinging or detonating, a richer mixture for gasser do not produce a signficantly lower CHT, so tuning by temp is dubious at best. EGT can be useful, but an infrared temp gun is a ineffective tool for measuring it.

Personally, I think the infrared temp gun is useful on gasser mostly as a tool to check for overheating in a cowled installation, and to check for temp disparity between cyl in a running multi-cyl engine. As is the case with instrumentation in general, just because you get a lot of "data" with the gun doesn't necessarily mean they offer a lot of useful "info". Try not to draw too many conclusions based on poorly qualified data is worse than if you don't.