Doesn't matter. The two important aspects of a chronograph, for most practical purposes, are that it's reliable (it captures every shot) and repeatable (the values don't change due to solar angle, shadow, PMS, whatever). It doesn't really matter if it reports the "real" fps, as long as it shows a 10 fps change as 10 fps. Which is to say that absolute accuracy isn't that important, but relative accuracy is. (Yes, please lecture me on accuracy vs. precision...) Oh, the third most important thing is that it not attach to the barrel. That's a complete non-starter.
As long as the relative accuracy is good, you have all the information you need to tune, etc.
I have owned perhaps as many as a dozen powder-burner chronos and it seems that most offsets are systematic. For instance, my pact chronos (3 units) were all consistently ~10fps "faster" than my Oehler chronos (2 units). The two Oehler and the three Pact were all consistent amongst each other. In any case, using a "ballistic pendulum" - which is the only way I know to test a chronograph available to those on a limited budget, I determined that the Oehlers were the 'gold standard'. CED chronographs match Oehler pretty well, but only in a full-on lighted box (not just lighted screens). I *suspect* (but don't really know) that the Caldwell is no better. The LabRadar (my current go-to for powder burners) also matches the Oehler. I use an Air-Chrony for my pellet rifles, but have never compared it to the others.
I'm not seeing a lot of comments about reliability / difficulty in different lighting conditions (where are my Pacific North West shooters?), but that's what would drive my decision.
GsT
