Biggest environmental factor for loss of zero?

JR86

JR86

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SW Ontario
Just curious if anyone knows anything about zeros changing day to day on rimfires. My bolt gun will have a 1/2minute point of impact variation @50 yards (up and down, not including side to side for wind) from range trip to range trip. Same range, same ammo from same lot. Temp, humidity, wind?

Thanks
Jeremy
 
Assuming that you have been able to rule out all mechanical factors (i.e. your stock is swelling and no longer free floating when humidity goes up), I would bet that wind is the the main driver behind your POI changes. The POI change shouldn't be only up or down but very much "all over the place". You can start collecting some data and try to correlate POI changes to a specific wind direction. I have found a lot of useful info in the "wind reading sticky" that has been saved in this forum. Let us know what you come up with.
 
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Yeah I ve read that thread about the POI shifting up with a 3 o'clock wind and down with a 9 o'clock wind. Was just curious if there was something I was missing. The rifle is a CZ 453 varmint. Walnut stock and generous area in barrel channel. Temp isn't changing too much. Was really just curious about relative humidity and pressure changes affecting .22s
 
Hotter or colder powder temperature can affect muzzle velocity, which can affect impact height. This temperature change can be from having the round sit inside a hot rifle chamber (also known as cooking a round) or can also be from the ammo being warmer or colder because of ambient air temperature. Unfortunately higher or lower muzzle velocity can not be directly associated with higher or lower impact heights because there are other factors at play, such as barrel harmonics.


I am also going to ask these other questions because I have experienced these issues myself:

  • Are you sure there is nothing loose on your scope mounting system? (one way to test this is to get lined up on a very steady rest at a target and try to push or flex the scope with your thumb or something, it is very hard to do this without moving the whole gun but I have found loose scope mounts with this method.
  • Are you sure that your scope can be trusted to hold zero?
  • Have you been cleaning the gun in between range trips? A dirty bore can print different than a clean one.
  • Are you shooting the gun the same way each trip? (eg. lead sled on day 1 and sand bags on day 2)

As stated earlier wind can also cause an elevation change in POI but without me taking the time to calculate the magnus effect for your specific .22 rounds from your specific rifle at 50 yards I'm going to have to say that is extremely unlikely even in high winds. Also sometimes wind can have up drafts or down drafts but once again unless the speed is rather high I don't think it would have much effect at this range but I could be wrong.

I would look at the shooter or the rifle system for the problem in this case, not the environment.
 
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A half minute change of zero at 50 yards? Unless you are an Olympic Target Shooter, that is hardly something to get your shorts up tight about.

Since you seem to have vertical stringing, I would look at the bedding, especially in the fore end of the stock. Make sure the action screws are tight.

Temperature, humidity, and even the position of the shooter's hand on the forestock can easily cause this. The OP is in southern Ontario, which is close to the Great Lakes and does have a high humidity factor in the summer, along with high summer temperatures.

A 3 o'clock or a 9 o'clock will not affect VERTICAL stringing too much but will affect horizontal bullet drift. A 12 o'clock wind tends to push bullets UP and a 6 o'clock wind pushes them DOWN. However, this is more for longer ranges and unless it is a terrific wind out there, the 50 yard affect would not be a lot.

Another factor most people do not take into account is LIGHT. The angle and direction of the Sun can affect shooting. Mirage, or heat waves off a hot barrel will give a false position of a target.
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buffdog
I agree the half moa variation from one shooting session to another is hardly consequential. However, because of the nature of rimfires and specifically those travelling in the transonic zone a vertical deviation from wind has long been observed and acknowledged. Full value winds (3o/c and 9o/c) have the greatest affect on vertical movement even moreso than 12o/c and 6o/c due to the bullet riding the wind or the wind riding over the bullet.
Right hand twist barrels will cause the bullet to impact left and high in a 3o/c wind and right and low in a 9o/c wind. For less value winds the deviation is correspondingly less and left hand twist affects in the opposite manner.
The vertical deviation from 12 and 6o/c winds can also and probably most often be in the opposite as you describe. A 12o/c wind will most likely increase resistance causing a lower POI and the reverse for a 6o/c - but again the vertical POI change is far less pronounced.
With high velocity CF projectiles the affect is far less pronounced and almost negligible.
 
So I'm running a bushnell Elite Tactical 10x40 on my CZ 453 Varmint rocking Eley Practice ammo. I can shoot sub half minute groups with centerfire rifles from a bipod with rear back which is exclusively how I shoot when tuning/testing rifles. Everything on this rig is nice and tight with blue loctite. The scope not only can hold a zero but also has awesome tracking.


I believe that my problem ended up being that the way my range is laid out I get a 6o clock wind almost exclusively. I'm not much concerned about a quarter inch variation at 50 yards... But I push my .22 out to 300 yards and a half minute at that distance can be the difference between a hit and a miss.

Thanks for the input guys
 
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