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Old 6th January 2010, 06:28 PM
greyskullnz greyskullnz is offline
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Join Date: Aug 2009
Location: Wanganui NZ
Posts: 268

Originally Posted by Robf View Post
I didn't check with everyone, but I think everyone I spoke to from the UK saw their POI rise in SA, and in fact it was a flatter trajectory... although the 25-30 didn't move, the far 55 yds saw around 1 MOA of rise, suggesting the pellet was flying flatter... which makes sense as the air at 4000ft is thinner... chairgun points to this but suggests less of a rise, however I didn't measure the humidity here and there

Certainly cold air is denser than warm, something that engine tuners and pilots know.

Humid air is less dense than dry air though... somewhere they probably cross over, but I don't know how to work that out unless someone has made a table up of air pressure vs humidity

So it would seem that perhaps temperature is not the key, but more air density. (i can hear all those ghosts of past days talking about falling shots over ponds... and I've seen shots rise really high low over the grass at Sywell in the heat, enough to take me and Neil Daniels out of a 40-45 yd kill in zero wind... and the rest of the course judging by the group of misses)

So the biggest drops would seem to be at sea level with cold hard frost sunny days in winter, and the biggest rise would perhaps be those humid hot sunny days... kind of bears fruit with my experiences.
You have those temp strips stuck around the front lens on your scope. Have you observed any trends from just watching them and ignoring atmospherics?

I'm interested to know what temp you have zero'd the rifle at, and what happens to your POI over a match, hypothetically when it starts cold and then warms up. (just watching the temp strips)

Is there a "band of temp" where all seems sweet, and extremities of temp where you observe some shift in either direction. Again, only observing the temp strips and ignoring atmospherics.

The reason I ask is because you hav the temp strips stuck around the location of the largest solid mass in the scope, the front lens, and this will take the longest to heat and cool/expand contract.

Just interested to find out if you have noticed anything by way of a repeatable trend, as I have. Not as dramatic as in SA, but at home here it is subtle yet still there in certain conditions which I have been able to recreate..

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