Fred, thanks for your report. Very, very helpful.
Barry, last week we tested an 80mm Swarovski,20-60x zoom) and a 100mm Pentax ED,26-77x zoom + 55X fixed).
We also had a NF 10-42X Nightforce riflescope and a 10-60X March Riflescope on hand.
We started at 06:30 am, literally at the crack of dawn.,The 100mm was noticeably brighter than the 80mm Swaro at this hour).
Our test target was an official 600-yard target with five 6mm bullet holes in it, 3 in white, one in blue, and one "double",overlapping shot) at the blue/white edge.
Viewing conditions were nearly ideal early in the morning. We were at a mtn range, 3000+ feet altitude. It had rained earlier in the week, clearing up the air. Conditions, initially, were windless with no mirage, and air was cool and dry.
The Pentax and Swaro at 60X could both resolve ALL 6mm holes at 600 yards, including the double. It was easier to see the holes with the Pentax set at 77X, however. The Pentax had slightly more contrast, while the Swaro was just slightly sharper,target lines were "crisper").
With the NF 10-42, 3 of the four observers could see all bullet holes at 600, but basically you couldn't tell the double was two shots. The one shot by itself in the blue was pretty hard to pick out. I wear contact lenses, and I couldn't see the shot in the blue with the NF.
At 60 power the March 10-60 Rifle scope could pick out all FIVE shots and you could tell the double was a double. However, the Swaro was sharper and the Pentax had more contrast. It was definitely easier to see all five bullet holes with the spotting scopes,either one). However the March was surprisingly good. the big problem with using the March is the 3.5" eye relief and ultra-small exit pupil. It's easier to use when sitting behind a rifle so your head doesn't move.
NOTE: Individual eyes make a difference. I wear corrective contact lenses, and the three other observers could all see better through the Nightforce than I could. Additionally, I could see better with my right eye than my left eye. It's interesting, not only do I have better resolving power with the right eye, but I see more contrast.
WITH MIRAGE
As the day went on, the wind arrived and we started to see mild to medium mirage. We started shooting on a white butcher paper target with 3" Orange stickers. I was still able to call shots in the white with the Pentax 100ED, and another observer was able to call shots in the white with the 80mm Swaro. Shots on the orange stickers were pretty easy to see even when mirage was bad.
POWER
While the NF resolved holes in the white at 42X, it was easier to see them with the March at 60x,but the exit pupil is really small at that power so eye/head position is very critical and hard to maintain).
I'd say you'd want AT LEAST 60X in a spotting scope and 80 power isn't too much. We cranked the Pentax all the way up to 77X and that was definitely better than 52X,fixed lens), or 60X,with zoom). Keep in mind the Pentax had a huge 100mm objective.
FOCUS
In order to get the most out of the spotting scopes, you had to focus very, very carefully. With the Pentax this was difficult because it has a single 1"-diameter focus ring. It was easy to go "too far" one way or the other with the focus ring on the Pentax. Also the Pentax was so long and heavy that it would shake or wobble a bit as you worked the focus.
The Swaro had a big, wraparound focus ring that is the full diameter of the body. This was much, much easier to focus precisely.
Concurring with Fred, I would say that you'd want a spotting scope that can deliver 60X at least, and I'd like to try 85X magnification on a 120mm scope...
This assumes, of course, that the task is to resolve 6mm bullet holes at 600 yards and beyond.