The best? Something like the old 100mm Unertl Team scope, or the new Kowa Bino team scopes like AMU is using. The first is long since out of production, and teh second are more than a little pricey. Emil Praslick was showing off the new accquistion to me some time back, and I like to fell off my shooting stool when he quoted the price!
Vapor trails are easily seen with any decent spotting scope. The new Kowa 82mms, the older 77mm models, or any others. Spent a lot of time in the military using the old issue M49 spotting scopes with very good results. For Rattle Battle matches (Infantry Trophy matches) the coaches are pretty well limited to using binos, since they don't have time to get behind each shooter if they're messing with a spotting scope. The important thing is that 1) the coach knows what he's looking for, and what he's seeing, and 2) that the coach is best situated directly behind and above the shooter, as closely as possible in each aspect. The first comes from time on the range an practical experience. The second does too, as he'll find out where he needs to be by spending some time in the coaching role.
The bigger scopes are easier to use, but this really is a task that relies more on the skills of the user, than the tools he's using.
Kevin Thomas
Lapua USA