Sorry to copy and paste this from the bbs but Rob may be able to shed a little more light on this and it does seem appropriate to this thread.
Cold temps and poi
Probably been done to death and I've just read a couple of lengthy threads on stb.
Conclusion seems to be that with some rigs there is a poi shift in temp ranges.
I zero'd a falcon fn19 yesterday at home and shot from inside the house ( so gun at roomish temp ). Scope was a cheap Hawke mil dot.
Today I drive to the Wedge to do some testing and it's obviously pretty cold. Prob just above freezing. Straight out of car and a quick 160 bar charge and straight to zero range. Shooting flat forearm prone and dead steady zero is bang on at 35 yards and cloverleafing. After several shots I try 40 yards ( all this is paper ). Bang on half a mil dot so happy with that. I then take several shots at the 45 yard shoot down/up target. Happy again ... 1 mildot.
I chat to the guy next to me for a few mins then decide to try the 35 yard zero again before going on the course. POI has moved to about 0.8 mildot low. I've only shot about 30 pellets so should still be in the sweet area. I ask one of the guys who helps run the Wedge and another overhears and says it's probably the cold affecting the scope. Suggestion is to rezero now gun / scope are cold. So I refill to 160 and rezero at 35 yards. It needs about 1/8 of a full turn.
All seems stable for about 20 more shots so I refill to 160 bar again and go on the course. Gun is shooting well and I'm knocking the targets down. To be fair to the Wedge they tend to set a course that can accommodate all levels of shooters so it's mainly 40mm kills out to about 45 yards ( there was one at about 50 ) with several 25mm kills. So I'm doing quite well. Then .... last 4 targets ( so I've shot about 26 pellets now as it's 30 shot course ). Shot 27 is a 40 yard 40mm kill. The previous 4 shots or so have been fairly close with huge kills so very forgiving. Anyway this 40 yarder I miss well low. Last 2 shots are 30 and 35 yards ... again full size kills so should be gimmes ( there's no wind ) ... both miss low. Cross hairs don't move through shot.
So that's the course finished. I pop back on zero range and again the poi is about 0.8 mildot low. I've only shot 30 pellets in a 19 inch tube starting at 160 bar. Still 120ish bar left in.
I can see why it shifted the first time because of the cold temp but would the gun/scope not have settled and then stayed where I rezero'd it? Would it have kept changing? Temp didn't change in the 2 hrs from me rezeroing in the cold to finishing the course.
A couple of guys then reply saying that temp affects the gun and scope etc.
I understand that the temp can affect the poi because of things described above. Many thanks for that.
So to compensate you leave the gun out to climatise for half an hour or so then rezero. I get that.
It's the second shift that is confusing me. Is it because the gun/scope etc hadn't finished shifting after the half hour or so of being out in the cold before the rezero? How long would you leave a gun out before a comp?
Obviously with it moving another 0.8 of a mildot over the course that could be crucial.
Like Rob has said ... I used to shoot a few times at Byley in Cheshire where some of the targets were over pools. I remember a chap there ( Terry Mann ?? ) who was always on about how the air low over the pools would drag the pellets down and you would have to give them more holdover.
I can try the FN over the chrono and see what happens re MV on a cold day and after various time intervals. I suppose that will tell what at least is happening with the gun.
Last edited by skires; 30th January 2012 at 02:35 PM.