Not having it in direct sunlight should reduce the problem of Index of Refraction caused by a blast... H's rifle was problematic to read, but it has no brake or anything beyond a tube. The IOR problem is emphasised by strong uni-directional light... (collimated to be precise)
Perhaps the muzzles need to be further back. From experiments down the club, with FAC gear, the higher the blast, the more the chrono has to go down range. When we were up to around 400 ft-lb we needed to be about 6ft away.
I don't think it's just the BFTA chrono, all seem to do it. I can't get a reading off a crombo on H's walther at all. I've seen some pepper pots give error'd readings. I tend to think that when you get a bad reading though it's way off rather than a smidge off. Obviously that's not a scientific fact, just opinion.
I've got a CED M2 with IR sensors, which should prevent some of the sunlight issues, but the fundamental is that pressure waves can distort the air enought that when any wave length of light passes through the waves they can make the chrono see them as an object and thus give a false reading. This is why you can see a higher MV a foot or so out over a measurement at the muzzle, because they have dispersed by then. But with higher powers or perhaps different muzzles, i suppose it's entirely possible these waves could travel further. I guess the trick becomes then in having a chrono that can't be shot...
No matter though, what error, tolerance or buffer or chrono used, there will always be an error possibility, and statistics say that it could mean someone is over or under. The problem is that we're trying to measure tiny things at high speeds, and technology is being pushed to acheive that.
If people know it could be hot, then they should make sure they are well under... and if the chrono is available to test beforehand, then this could reduce the amount of those that fail. I understand perfectly that a day could be ruined, but if it's that important, then do some prep work beforehand to mitigate the possibility. You can only do as much as you can, and so can the BFTA, and nothing is perfect i'm afraid.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep