Re: what does the weather do to a rifle.
1/4 MOA =
1/4" at 100
1/2" at 200
3/4" at 300
1.0" at 400 and so on.
At what distance it becomes a problem is determined by the size of the target, the rifles capability, the shooters capability with the rest he has and the weather conditions he is currently dealing with.
The rifle is capable of keeping every shot within .5 MOA of point of aim (POA), or all shots within a 1.0 MOA circle centered on your POA.
Now enter a little shooter induced error and you may now only be able to keep all shots within .75 MOA of POA, or in a 1.5 MOA circle.
Temperature has increased 30 deg F. at mid-day, up from 60 deg to 90 deg, and now changes your POI from POA, say .75 MOA.
At the same time a low pressure system has moved in and thus atmospheric pressure has dropped from, say 29.50 to 28.50 hg in the last 6 hours. This may mean about .25 MOA.
If your ammo is not in your chest pocket or near your body some other place and at the same temperature you sighted in with, you have one more variable to contend with... a MV change. We will say it is a non issue and remains consistant.
Your target is at 300 yards.
We know the rifle is capable of 1 MOA at 300 yards, or 3" groups consistantly.
We know the shooter is only capable of holding groups consistantly inside of 1.5 MOA at 300 yards, or 4.5" with this previously tested field rest at 60 deg F. and a 29.50 BP in zero wind, his logged sight in conditions.
If he holds, or dials 3 MOA up elevation for the new 300 yard zero, POI will be .75 MOA high, or 2.25" as a result of the 30 deg increase in air temp.
Air density has also decreased due to the reduction in atmospheric pressure and POI will be high by .25 MOA, or .75" at 300 yards.
You subtract .75 MOA elevation for the air temp change, now at 2.25 MOA.
You subtract .25 MOA elevation for the BP change, now at 2.0 MOA.
POI would have been roughly 1.0 MOA higher, or 3.0" than expected if these two variables were not accounted for. Remember, this is with a stable and predictable MV unaffected by the temp change.
If the same shot was to be taken at 600 yards, that same 1 MOA error, if not accounted for, would have meant a POI shift of 6.0".
Now the question remains; What is your target's size and what POI to POA deviation is acceptable. Also, and just as important, at 600 yards, am I still able to keep all of my shots consistantly within a 1.5 MOA circle and even more importantly, each shot within .75 MOA of my POA.
The fact of life is your ability to maintain that will without a doubt degrade as range is increased. Learn what you can maintain at various ranges, be honest with yourself about it and you will have a solid baseline from which to work from and progress.