I'm not 100% certain, but I don't think that Hubble quality will do it either.
The deal is that there is a lot of optical noise in the air (moisture, mirage, etc), so unless the conditions are just so, I think that you are wanting something that just can't be had. Maybe if you use those shoot-n-see targets ... then I think a nice bright 80mm objective spotting scope in the 20-25x range would be sufficient at 300 and might even get you to 500.
Saands
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