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 Post subject: Temperature & POI
PostPosted: Sun Sep 23, 2018 9:53 pm 
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Joined: Sat Jun 22, 2013 1:21 am
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Location: Toronto, Ontario
Last night at the range I had the oddest thing happen to me while shooting the TX200 .22 cal. When I arrived around 7:30 PM. I immediately zeroed my rifle at 20 yards using a brand new tin of JSB Exact Diabolos, 15.89 gr. I shot for about 2 hours and noticed my shots were off at various distances downrange as the holdover and holdunder points were no longer accurate. So I went back to 20 yards to see if there was a shift in the zero. Turns out the shots were landing about a quarter of an inch high at 20 yards(zero point) Up to this point I didn't tweak the scope in any way or fiddle with the scope rings/mount. My hold and body positioning was the same and consistent throughout.
I know it wasn't the scope as it's a brand new Hawke Airmax mounted on a brand new one piece Sportsmatch rings. I've been shooting with this set-up the last few weeks with no issues.
The one thing I noticed throughout the evening was the drop in temperature from 7:30 to 11:30 when I left. When I arrived, the temp. was hovering around 64 degrees F and started to get cooler as the night progressed. By the time I was leaving, it was 48 degrees F. Mind you, the range that I shoot at has fans behind the ports that blow the smoke and lead downrange. This made the evening a very cool one as there was a constant draft behind me. Even the barrel and cocking arm of the TX was cold to the touch.
Nevertheless, after Re-Zeroing the rifle, my shots for the next 2 hours of shooting was back on target. I even chronied the gun today and it's consistent to the readings from the past recordings. The only thing I could think of is the drop in temperature might have some effect on the mainspring and lube within the mechanics of the gun. Maybe the velocity changed, resulting in a shift in the POI.
Anyone out there with a springer experience something like this?

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 Post subject: Re: Temperature & POI
PostPosted: Mon Sep 24, 2018 6:11 pm 
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Joined: Wed Oct 17, 2007 3:27 am
Posts: 2549
Location: Winnipeg, Canada
I don't pay that much attention to my springers. The one that I am semi serious with seems to hold a consistent zero regardless of temperature.
1/4" at 20 is over m.o.a..

Kind of curious what lube is on the gun. Only quick thing I can come up with for a drop in velocity due to cooling...
I would test this against the chronograph to be sure it's not something else.
My springer experience is still on the limited side...

edit: just re-read your velocities are still consistent with past performances.
Add the cold temperature variable over the chronograph and see what happens.

-D.S.

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 Post subject: Re: Temperature & POI
PostPosted: Mon Sep 24, 2018 9:02 pm 
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Location: Toronto, Ontario
Doc Sharptail wrote:
I don't pay that much attention to my springers. The one that I am semi serious with seems to hold a consistent zero regardless of temperature.
1/4" at 20 is over m.o.a..

Kind of curious what lube is on the gun. Only quick thing I can come up with for a drop in velocity due to cooling...
I would test this against the chronograph to be sure it's not something else.
My springer experience is still on the limited side...

edit: just re-read your velocities are still consistent with past performances.
Add the cold temperature variable over the chronograph and see what happens.

-D.S.


Thanks for chiming in Doc! After doing some research on the internet and Youtube, it turns out that temperature and humidity can play a role in the action of a spring piston gun. I read in other forums that cold temperatures equate to dense air resulting in a higher velocity at the muzzle due to compression of the dense air. On a Facebook, Air Arms Forum, two fellas with the same gun experienced the same thing while shooting in cooler temperatures. One shooter said he was 5 clicks high at 20 yards when he shot in the cool morning versus the warm afternoon. I was also 5 clicks high @ 20 yards. Looks like I'm going to have to come up with H/O & H/U data points for 3 temperature conditions(Cool, Warm, Hot). I'll have to re-zero or check the zero once the temperature changes through a long shooting engagement.
I saw 2 Youtube videos demonstrating the lower muzzle velocities attained while shooting in cold temperatures versus shooting in a warm environment with a spring gun.

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