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Old 27th September 2011, 11:57 AM
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RobF RobF is offline
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Join Date: Mar 2010
Member of: Southampton Buccaneers, Parkstone, South Dorset
Location: Poole, Dorset
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Default The Kestrel/Density Altitude/POI Shift thread

Cue rolling eyes from the back of the class of something else technical i've purchased for shooting, but the phenomenon of POI shift which seems to correlate with weather has had me head scratching for years. Aside from the impact temperature can have on a scope's ability to tell a true range, or the effect it has on an airgun's ability to deliver a constant muzzle velocity, there seems to be the possibility atmospherics have something to add to the muddle that can send POI way off.

In Italy the springer delivered shots over the crono with less than 3ft/s variation total over 3 days,
the scope never ranged over or under, yet I saw 1.5 MOA of shift during the comp (meaning the scope would have had to misrange by 10yds in some cases or invent a new zenith in others)... either the barrel was raising in the heat and dropping in the cold(er) air or something else was happening.

Pete M put me onto a cheap source of the Kestrel 4000, a few years ago these were 400, now commonly found for 250, at 1/2 price on the bay it seemed like a good deal. These combine humidity, temperature, barometric and altitude data to come up with a benchmark figure called Density Altitude. This is because although air temperature changes density, air pressure can change it the other way, which means there can be a time where density can be unexpectedly denser or lighter especially when altitude as well varies. The idea that a cold damp morning has denser air than a hot one may not always be true. So Density Altitude gives us a reference point to work with, and the Kestrel can measure and calculate this.

It seems to have impact in long range full bore shooting, is a useful link, and there is a commercial outfit custom building them for specific loads in the US. As FT seems to be a short long range shooting competition, it's plausible the same effect could happen to FT shooters. Whilst some shooters never seem to have a problem, I suspect that those that have done a few visits abroad have seen movement, and some who live in wildly changing climates might see it far more often.

So hopefully this thread will be a chance to correlate the info out there and scale it down for airgun purposes, and see if the effect is real and has an impact, or is another bit of technobable that can be ignored, or a bit of both.

So far though, the Kestrel seems simple to use. The manual is tiny and about 30 secs of reading you are off and using it. So I am now praying for some wildly varying weather and some shifting POI to accompany it... so expect good weather and stable zero points for a while
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