Re: mirage question
...not an expert here, but here's my take which I'd love to get feedback from the experts on... in terms of "learning to dance with it" as royinidaho puts it...
Unless I'm mistaken, mirage doesn't move bullets, wind moves bullets. And, mirage is a reflection (pun intended) of what the wind is doing.
Boiling and/or changing is a big problem
However, if you observe mirage closely along with reading the wind, and if it remains constant/consistent in one direction, then your POI may have shifted. But, it shouldn't have a huge impact on your group size relative to the distance you're shooting.
So, depending on what the mirage/wind was doing, that could be a possibility.
But like others stated, there are many other variables that affect group size.
I would think that an estimate of the min-max wind on that day run through a ballistics program with a correct BC for your bullet and MV should give you an order of magnitude as to how much the wind might've moved your bullet around.
Unless there were up/down drafts, which is less likely at 100yds than 1k, then it's not likely going to account for huge vertical dispersion. ...except possibly as pertains the the right hand twist and some slight upward/left movement with a 3 o'clock wind and a slight downward/right shift with a 9 o'clock wind.
I digress, but... My 15yo son was shooting at 500 yds about 2 weeks ago and one of the other shooters on the line pointed out that focusing your scope at 500yds, you could see the mirage moving from left to right and focusing at 300 yds, it was running the exact opposite direction. It would do that for 10-15 minutes and then swap directions.
As a result, his group size opened up about the right amount by my SWAG (142gr SMK, .595 BC, 2825mv, 500yds, 5mph, ~7") and most of the dispersion was horizontal. I had him aim dead on every time and not try to compensate so that we could see the effects. But, if he can learn to "read" it well in real time, then he should be a much better shooter for it.