I generally avoid getting into these discussions, as I still don't feel like my data set is definitive even after spending two years working with tuners. I have some theories, insights, and observations, maybe I can share a little.
It would seem like the reverse is more likely to yield good results. That is, if the tuner is adjusted to shoot well at 100 yards, it'll do well going back to 50. What seems to be working at 50 yards might not hold at 100. Sometimes, a good 50 yard tune does, in fact, hold out to 100 or even 200. Explanation for the phenomenon is not so cut and dried/black and white, which leads into your final question...
I don't see a consensus on that. I see two very divided camps religiously adhering to their theories. Consider one simple fact for a moment: no official 25 shot competitive target ever shot demonstrates "positive compensation". The vertical spread due to the muzzle velocity ES of the very best lots of ammo available is apparent in all targets, to the tune of 0.150" to 0.170". How come nobody has been able to "compensate" this vertical out over several decades worth of shooting with muzzle devices/tuners?
I see some validity to both "tuning" a barrel, and "stopping" the muzzle. On the "tuning" side, you know that your naked barrel will shoot well with certain ammo under certain weather conditions. Centerfire shooters can fine tune their handloads to get some extremely good long range accuracy without a tuner, but may have different loads for cool and warm weather to do so. You cannot adjust rimfire ammo, but, through the use of a "tuner", you can adjust how your barrel responds to that ammo. What I have observed, is that with a Harrels tuner and a weight set, for a given ammo, a "tune" can be found with no weight added, a light weight, a heavy weight, and a light/heavy combo. This "tune" only works for that particular ammo, in the weather conditions of the day. Change ammo, need to re-tune. Different day, re-tune (but maybe not too far off from the previous setting). After a time, I began to feel like I was chasing my tail with "tuning".
My interpretation of my data over the last several years favors the "stopped muzzle". Goodness, it takes a great deal of shooting to thoroughly vet out tuner settings, but I find myself consistently coming back to using no weight and being in the setting range between 200 and 250. My 1/4" success was shot at 238, and a very near attempt with the worst group at 0.265" was shot at 233. Within that range, it is difficult to say there is a noticeable difference at any random setting, it all appears to be about equal. I can change ammo and do not feel the need to make any adjustments. Temperature seems to have no effect, what setting that was good in the warm is good in the cold. I haven't shot at 100 for some time, but perhaps I can take it out again and see how setting 238 does with the R-50 ammo I used for 1/4" and report back if any adjustments were needed. I'm not yet prepared to come out and definitely support the "stopped muzzle", but I will say I am leaning in that direction. This future 100 yard test will be interesting, and I think I should also do it over a chronograph. That'll reveal if the muzzle is "stopped", and the vertical is attributable to MV variation alone (which seems to be impossible to "compensate" for).
