Thought I would ask here. I was wondering do temp, humidity. or other conditions effect the POI significantly at 300 yards. Reason for asking is I was out shooting groups and I noticed my POI was 3 inches lower than the last outing. Groupings were fine windage was fine but the groups printed three inches lower. Probably about a 10 degree difference in temp between the two outings.
POI change
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That is interesting. I am learning a lot in this last while. For the last 40 years I have been loading and shooting for basic hunting rifle accuracy. I took the plunge and started to get into more precision shooting. Lots to learn and fun doing it.
And yes you are right one day was sunny hot with a bit of a wind. The other day was cool cloudy and no wind. Never thought it would have that much of a difference at 300 yards.
And yes you are right one day was sunny hot with a bit of a wind. The other day was cool cloudy and no wind. Never thought it would have that much of a difference at 300 yards.
significant pressure and temperature change can cause 1/4moa shift. change in wind might add 1/4moa. so 1/2moa shift at most.
If the optic was not bumped, check headspace of your ammo and fired case. check bullet seating depth.
few 1/1000 inch increase in headspace and bullet seating depth can decrease velocity.
If the optic was not bumped, check headspace of your ammo and fired case. check bullet seating depth.
few 1/1000 inch increase in headspace and bullet seating depth can decrease velocity.
Stress in the scope tube can cause changes in POI. This stress can be introduced during mounting.
Was a very interesting experiment done by an interested 'gun nut' years ago that I recall. He mounted scopes in mount that was very solidly bedded to a very heavy concrete bench rest surface. He adjusted the reticle to coincide with a point on a target and set out to discover how stable the reticle would remain under external forces and if it returned to zero after power changes and adjusting for windage and elevation.
One thing he discovered, with the scope held stable, was that simply placing a rubber band around the objective bell and exerting just the force of pulling on the band around the objective caused the reticle to shift on the target. Sometimes when he released the rubber band the scope returned to 'zero' ... sometimes it did not. He also tapped the scope with a rubber mallet and found a similar reaction.
He found the 'return to zero', after physically displacing the objective, to be a function of whether there was some tension on the scope from the rings. He concluded that much of this tension could be relieved by grabbing the objective bell of the scope (while mounted) and trying to firmly oscillate it in the rings before shooting. I presume longer scopes are potentially more prone to this phenomena.
look at the scope at 3:04 .. of course this is extreme but if the tube is under stress from the mounting -- firing recoil may release this stress with the result that the reticle will come to rest at another POI ... then of course there are the opposing internal springs in your scope W&E adjustment that will react as well ... sometimes unpredictably.
Was a very interesting experiment done by an interested 'gun nut' years ago that I recall. He mounted scopes in mount that was very solidly bedded to a very heavy concrete bench rest surface. He adjusted the reticle to coincide with a point on a target and set out to discover how stable the reticle would remain under external forces and if it returned to zero after power changes and adjusting for windage and elevation.
One thing he discovered, with the scope held stable, was that simply placing a rubber band around the objective bell and exerting just the force of pulling on the band around the objective caused the reticle to shift on the target. Sometimes when he released the rubber band the scope returned to 'zero' ... sometimes it did not. He also tapped the scope with a rubber mallet and found a similar reaction.
He found the 'return to zero', after physically displacing the objective, to be a function of whether there was some tension on the scope from the rings. He concluded that much of this tension could be relieved by grabbing the objective bell of the scope (while mounted) and trying to firmly oscillate it in the rings before shooting. I presume longer scopes are potentially more prone to this phenomena.
look at the scope at 3:04 .. of course this is extreme but if the tube is under stress from the mounting -- firing recoil may release this stress with the result that the reticle will come to rest at another POI ... then of course there are the opposing internal springs in your scope W&E adjustment that will react as well ... sometimes unpredictably.
Hold on, are you pulling my leg? Sun to cloud cover can affect accuracy by that much?
One thing my old physics classes taught me was that the changes of density of a medium effects the refraction of light through the medium. ( The old spearing a fish off a dock demo). Therefore if the air density changes from day to day then the angle of refracted light will change. This would mean that what you see is not exactly where it is in reality. This would result in a different POI. I just never knew it could be to the extent stated.
This and mirage effect as well.
Or of course the most probable cause was shooter error. ( Too much coffee and crappy eyesite lol...)
This and mirage effect as well.
Or of course the most probable cause was shooter error. ( Too much coffee and crappy eyesite lol...)
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We always like to blame the equipment, but the first thing we need to do is look at ourselves.
Difference in cheek pad pressure, bipod loading, trigger control, trigger grip, pressure into the butt pad/stock, how you line up behind the rifle, follow through, etc. will all show up in your POA/POI. Any difference in any of the above, especially a combination of the above, will result in POA/POI shift.
It's an easy thing to do from one day to the next. Perhaps you are stressed and applying more pressure to the gun. Maybe you are rushed as you want to do some load development before an engagement later in the afternoon, and you are rushing getting behind the rifle and getting shots off. Maybe it's colder and you are wearing thicker clothing, and aren't aware of the difference it's making between the interface between the shooter and rifle. Maybe your muscles are cold and or tight, more so then the previous outing, resulting in a difference with how you interact with the rifle. Maybe you were shooting from a bench the last outing, and this time you are shooting prone and that body mechanics difference wasn't accounted for, and the fundamentals aren't applied as they were the previous outing (or vice versa).
It's why nailing down the fundamentals, and applying them consistently day to day is really important. While it's easy to blame the equipment, a lot of problems start at the shooter.
Difference in cheek pad pressure, bipod loading, trigger control, trigger grip, pressure into the butt pad/stock, how you line up behind the rifle, follow through, etc. will all show up in your POA/POI. Any difference in any of the above, especially a combination of the above, will result in POA/POI shift.
It's an easy thing to do from one day to the next. Perhaps you are stressed and applying more pressure to the gun. Maybe you are rushed as you want to do some load development before an engagement later in the afternoon, and you are rushing getting behind the rifle and getting shots off. Maybe it's colder and you are wearing thicker clothing, and aren't aware of the difference it's making between the interface between the shooter and rifle. Maybe your muscles are cold and or tight, more so then the previous outing, resulting in a difference with how you interact with the rifle. Maybe you were shooting from a bench the last outing, and this time you are shooting prone and that body mechanics difference wasn't accounted for, and the fundamentals aren't applied as they were the previous outing (or vice versa).
It's why nailing down the fundamentals, and applying them consistently day to day is really important. While it's easy to blame the equipment, a lot of problems start at the shooter.
Kudos to the op for noticing. Many would not.
Something tells be that you are smarter than the average bear and I doubt very much the problem is you.
The problem you noticed is quite normal and is caused by one of two things...
Either the close range zero is not stable and moves from one day to the next, or it is atmospherics of some sort.
You can test for short range POI shifts by shooting a group close like 100 yards and see if the offset is constant... If it is then look to the rifle... possibly bedding or action screw torque
If your 100 yard zero is correct, then look to atmospherics.
As stated earlier, lights up sights up... but understand that assumes the light source is above you... look where the sun is before you assume refraction is up... refraction is either toward or away from the sun... not simply up or down.
ON FLAT OPEN GROUND... A head wind causes rounds to hit low... a tail wind causes rounds to hit high.
A wind from the right causes rounds to hit high and left
A wind from the left causes rounds to hit low and right.
In hilly terrain you need to account for updrafts and down drafts based upon hills... or even tree lines on otherwise flat terrain.
Rounds go faster is warm thin air and slow down more quickly in cold thick air.
Temperature can also affect your muzzle velocity.
Also look into density altitude.
If you don't have a Garmin 701 or Kestrel with AB on it, you can download an app called BallisticsArc... you can pull weather data from a nearby airport and produce ballistics... compare your results to that and you may very well find the app anticipates the condition you noted.
Something tells be that you are smarter than the average bear and I doubt very much the problem is you.
The problem you noticed is quite normal and is caused by one of two things...
Either the close range zero is not stable and moves from one day to the next, or it is atmospherics of some sort.
You can test for short range POI shifts by shooting a group close like 100 yards and see if the offset is constant... If it is then look to the rifle... possibly bedding or action screw torque
If your 100 yard zero is correct, then look to atmospherics.
As stated earlier, lights up sights up... but understand that assumes the light source is above you... look where the sun is before you assume refraction is up... refraction is either toward or away from the sun... not simply up or down.
ON FLAT OPEN GROUND... A head wind causes rounds to hit low... a tail wind causes rounds to hit high.
A wind from the right causes rounds to hit high and left
A wind from the left causes rounds to hit low and right.
In hilly terrain you need to account for updrafts and down drafts based upon hills... or even tree lines on otherwise flat terrain.
Rounds go faster is warm thin air and slow down more quickly in cold thick air.
Temperature can also affect your muzzle velocity.
Also look into density altitude.
If you don't have a Garmin 701 or Kestrel with AB on it, you can download an app called BallisticsArc... you can pull weather data from a nearby airport and produce ballistics... compare your results to that and you may very well find the app anticipates the condition you noted.
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At longer distances (300-1000 yards) I've never noticed POI to move more than 1/2 MOA total from heavy cloud to bright sun. Maybe it has, but I just don't recall. I just go with 1/2 MOA when I see a change.
In F Class, If you notice a cloud come over the range and don't adjust down for it you will drop points. Then when the cloud rolls by and the range gets bright again, you got to come back up. Then the trick is when the range is only dark half way... you split the difference. It's a leap of faith and you learn to take it and trust in it over time.
You can easily watch this happen by watching mirage refraction through your scope with the rifle supported. I made a tripod mount where I drilled and tapped a 1/4-20 hole into a cheap Chinese pic rail. I mount it on the tripod and use a QD scope mount to put my 24x scope on it. You can watch the target move and keep track of it if the tripod is short and stable.
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Just noted an interesting thing after looking at targets again . I had three groups each from a different load. The first two the POI was about the same as shots from a previous day but the third which was the one with more powder than the other two that was lower than the previous shots from an earlier day. Also all three groups were about the same elevation. BTW windage was spot on for all of them.
The brass I was using was on it's last load and is being tossed. Guess I will get some new brass ready and go back at it. It has been a real eye opener going from loading for old hunting rifles to loading for a precision rifle. Minute of pie plate to MOA. Measured the group and it was .6 MOA. I wonder what this rifle will do when the shooter gets it right.
The brass I was using was on it's last load and is being tossed. Guess I will get some new brass ready and go back at it. It has been a real eye opener going from loading for old hunting rifles to loading for a precision rifle. Minute of pie plate to MOA. Measured the group and it was .6 MOA. I wonder what this rifle will do when the shooter gets it right.
One year shooting the DCRA Matches in Ottawa we had very strong head winds all week long. Strong enough that your eyeballs were dried out after a 10 round match. At 300m everyone was finding that they needed to come up at least 1 MOA from their regular sight settings. At 500 and 600 is was 1 1/2 MOA and at long range you had to add 2 MOA to your regular sight settings. 1 MOA doesn't sound like much but it is about 3 inches at 300m. 2 MOA at 900m is almost 20 inches. Then there was wind and changing light conditions to deal with on top of everything else you have to deal with shooting iron sights.
Shooting is always a lot more fun and interesting when the wind is blowing. If the wind isn't blowing, everyone it tied for first place.
Shooting is always a lot more fun and interesting when the wind is blowing. If the wind isn't blowing, everyone it tied for first place.
It is far more likely that you induced a positional change.
Begin your next shooting session with dry firing. Get set up behind the rifle, run the bolt and focus on the target. Close your eyes and start your breathing cycle followed by your trigger pull. After the click, open your eyes. If your reticle is not still centred on target, you are inducing pressure and that will cause a shift.
Begin your next shooting session with dry firing. Get set up behind the rifle, run the bolt and focus on the target. Close your eyes and start your breathing cycle followed by your trigger pull. After the click, open your eyes. If your reticle is not still centred on target, you are inducing pressure and that will cause a shift.
