Results 1 to 8 of 8

Thread: A Question For The Experts...

  1. #1
    Join Date
    Jul 2009
    Location
    San Antonio, TX - Flathead Lake, MT
    Posts
    183

    Cool A Question For The Experts...

    We shot our last match of the season yesterday Oct 1...The conditions were NO wind which was nice, but then it was too cloudy and we had a drastic drop in temperature from 62-64 F to 32 F before we had a little snow (.1 of an inch or so) ...All of this happened in less than 1 - 1.5 hours...

    I was shooting a HV RAW TM-1000 and with these conditions and not so good shooting I ended the very good trend and success I had with this gun this short season, nevertheless I was not able to figure out what happened or why and let me explain:
    • Temperature was 64 F when we started shooting the first card...I had a chrono for adjusting and checking velocity/consistency after cleaning and doing some maintenance on hammer, hammer spring, valve, etc...
    • Adjusted to usual velocity with sighters before shooting card and the velocity consistency was there at 64-65 F, the gun was dead on.
    • Temperature dropped to around 47 F while shooting card number 1 ...The natural light dimmed rapidly with clouds getting darker (first 20 minutes), velocity only dropped about 5 fps with this 18 degree drop in temperature...Gun started to shoot a little low and slightly left when the very little wind we had was coming from LEFT at around 2 MPH. Under normal conditions and 1-3 MPH winds this particular gun remains dead on and it has a very large sweet spot (velocity)
    • Temperature dropped even more during 3rd card and we had a little snow at 32 F...velocity on gun remained the same as when 47 F...So from 65 to 32 F velocity dropped an average of 5-7 fps..With sized-weighted pellets this RAW usually shoots within 2-3 fps very consistently....
    • At 32 F when it started to snow, the gun corrected itself from shooting slightly low and to the left and came back to shooting dead on (as sighted in)...The flags indicated about the very same conditions for all targets so wind was never an issue (very little wind at 1-3 MPH coming from 9-10 o'clock)


    I haven't been able to figure our what happened (shooting low to the left) and I wonder if the gun was a actually not affected and it was the scope what changed the POI while the internals adjusted to the temperature???...I unfortunately didn't check the parallax when the gun was shooting low so I don't know if this could've been the issue...
    The scope is a Leupold Comp 40 X with matched lens from the custom shop and has been very repeatable, reliable and very stable when used for BR and for Field Target shooting at different temps (I have used the very same tape on the parallax wheel and have never had a parallax issue during the winter-summer seasons).

    Note: Today I checked the gun with another full card at 55 F...The gun is dead on with no issues and velocity is back to within 2-3 fps from how it was at 65 F yesterday.

    Any ideas or insights from the experts?

    Best regards,

    AZ

    PS. We will not be shooting in MT until late April early May next year (snow allowing)...I will be heading south to warmer temps. soon, getting old and cold weather don't get along very well anymore
    Last edited by AZUARO; 10-03-2018 at 01:08 AM.

  2. #2
    Join Date
    Aug 2005
    Posts
    724
    I am thinking just the light changes were giving you the fits. yep light changes do affect scopes, not as pronounced as irons but its there just the same. I know that my zero is going to be different at 8am than at 12 pm and again change around 4 pm just from the sun traveling over head. There is an old saying about light conditions and your sight picture- at the moment my old noggin can't put it together, I think it went sun up sights down and sundown sights up , I could have that reversed. at any rate as the light intensity shifts so does your zero enough to miss that 1/16" dot at 25 meters. at 600 yards it can take you out of the x ring which would be a 3" shift if you are dead nuts centered. In silly wet the sun affect is quite pronounced. Even with cloudy conditions you still have mirage to deal with. Then we can get into atmospheric density shifts just to add a bit of icing on the cake. I have extremely sensitive flags that will pick up thermals that most flags would not even quiver in. Even at 37 degs and cloudy there are still thermals. Now if i could just put all this together during a match it would be great but I always seem to forget parts along the way.

  3. #3
    Join Date
    Dec 2017
    Location
    Michigan
    Posts
    5
    AZ, Iím by no means in your league, so forgive me if I sound foolish but things to think about, is it a sleeved barrel? And or are the scope mounts of a different metal than the aluminum of scope and action?, also is stock walnut or laminate these can all effect P.O.I. I live in MI. And have shot in some similar drastic Temp changes within a shooting session( normally with gusts of wind, front moving in!) but Iíve experienced similar issues w/my RAW, mine is all stock though, just throwing un-educated ideas out there!

  4. #4
    Join Date
    Feb 2013
    Location
    PNW amongst the Redwoods
    Posts
    139
    AZ
    i know for a fact that temperatures affect harmonics
    i'm a beginner and i have seen it
    is this what you are hinting at ?

  5. #5
    Join Date
    Feb 2003
    Location
    Tennessee
    Posts
    4,927
    I don't think anybody has actually studied this well enough to know but either the rifle or the scope or both changed with the temperature. I doesn't take much change to get the results you saw. Blades mentioned the light and that very likely played a part as well. Why it is and what it is doesn't really matter in the least as long as you know it can happen. Pretty sure that's one reason scopes are adjustable. Thankfully, it doesn't happen much between shots...but it can be "happening" during a 20 minute target. I don't know anything about air rifles but the effects you speak of seem to be magnified when shooting a 25 bull rimfire target. Often, where you hold in the beginning changes quite a bit near the end....same condition...or seemingly the same.

  6. #6
    Join Date
    Oct 2015
    Location
    Zephyrhills, FL
    Posts
    29
    With change in air temperature, air density changes, humidity changes, and velocity probably changes too resulting in change in drag and who knows. Maybe the brilliance of the sun or lack thereof affects your POI too. Further, the temperature change will reduce the bore diameter and alter the pellet drag/velocity. This can go on and on.
    Last edited by Paul in Zhills; 10-04-2018 at 02:41 PM.

  7. #7
    Join Date
    Jul 2009
    Location
    San Antonio, TX - Flathead Lake, MT
    Posts
    183
    Thank you all for your very positive good replies...

    Scope, rings and action block are all aluminum with similar or very close thermal expansion coefficients....Stock is French Walnut very well sealed and prepared for BR and the barrel is not the original Poly but has the original RAW ported sleeve that came with the Poly barrel and it is bonded perfectly centered with bedding acraglass...

    I shot a 250 first target paying attention but not struggling much, but then I struggled much to keep the pellets close enough to the 10 ring for scoring 10's or X's (next 3 targets),,The conditions were optimal with no wind...The temperature dropped 30 degrees in between the first and sixth target (1.5 to 2 hours) ...On the 5th target at around 35 F the gun became alive again and was dead on (shot a 249-15) and the very last card on the second relay the gun shot a very good 250-18X card...I used a chrono for all shots because after working on some internals it was necessary for checking consistency (I usually get stats from 100 or more shots).

    The issue I haven't been able to even get close to figuring out is why is it that the gun shot very well at 65 and 32 F but didn't shoot at temperatures in between (shot low and to the left) ...The first card had good lighting conditions, targets 2 to 6 were shot with darker conditions but the good targets (5 and 6) tell me that the issue was most likely not the low light.

    What Dick says (Harmonics changing with temperature) is one variable that intrigues me and it could very well be the problem, but I don't have enough data nor experience to support this thesis and I don't even know if the problem in my gun is repeatable or if it is 100% dependent on temperature...If you have more insights on this Dick, please step in and illustrate us.
    Air density and humidity definitely changed as temperature dropped, I could feel it, nevertheless the chrono indicated an average of 5 fps or so variation in velocity for all cards.

    Again, thank you all form your inputs...I am one of those nuts who want to have an explanation for everything (learning) but many times some issues are hard to explain.

    Best regards for all,

    AZ
    Last edited by AZUARO; 10-05-2018 at 04:29 AM.

  8. #8
    Join Date
    Feb 2003
    Location
    Tennessee
    Posts
    4,927
    Pretty sure this is just one of them "things". You can come up with an explanation but it will likely be wrong. The deal is that you and your rifle may not experience such an incident as many times as others and all will be good in the end.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •