The best ive looked through is a Swaro Z6...if i had money, id have thm on most if not all my hunting outfits.
I havent seen through the 6500's, but no scope wound up to that Mag is going to look "great" in either low light, or a mirage.
Personally, i dont see how you could set the paralex on a scope accurately (using your vision for reticle movement) when your in a mirage...there is no way to tell if the movement you are testing is the mirage or your head movement.
Also, the reticle appears not as sharp as it will when not seeing through a mirage...thats just the illusion of what we see & how light works.
If the reticle was not clear, did you set the ocular correction for your eyes every time you shot it...?.if so did you do it correctly...?..& did the other guys change it...?
Im not being smart, but its hard to know what youre seeing as said above
I guess the other option is if something has come loose within the scope...I dont know how scopes are constructed these days, but in the old days the reticle were literally a wire & ive seen them "dislodged" before & appear blurry because theyve shifted within the scope.
But arent they etched these days...?...i dont know, but to me that sounds like theyre etched into glass element...in which case, i guess if an element with the etched reticle has come slightly adrift, it will make the reticle unsharp & unable to be focused sharp for you eye...
You say "reticle shimmer"...to me a shimmer is a reflection that somewhat "dances/sparkles"-like the stars-say planet Venus at night--, but the object its reflecting off of does not move--its just the atmosphere (mirage effect) causing the "shimmer/sparkle"...
If this is the case & the reticle is "shimmering", its just light hitting the etched part of the glass & reflecting off of it...ive seen that before...that coupled with the mirage from behind--like a backlit situation--may cause the reticle to appear like it "shimmering/dancing" a bit...
Thats all i got...
If the scope is mounted with a pic rail...set up another scope in seperate mounts & sight it in on the rifle...
Do a test with both--you dont need to re-zero each scope in when you put it back on the rifle, they will be on paper when you put the scope back on, so just shoot 2 groups with each in same conditions on same magnification..
.just dont do the test in a mirage...!!!!!
The printed results will tell you if one scope is foobarr'd...
The man who knows everything, doesnt really know everything...he's just stopped learning...