As far as the laser guns in "The Cage/"The Menagerie," we don't really know how those operates or what those three emitter barrels actually represent.
True. But amusingly enough, the barrels
are used rather consistently. When the elevator door slams on our heroes, trapping Pike inside, Spock fires at the door from the longest and narrowest barrel. Tyler uses a squatter barrel at first, but then adjusts his gun to use the long barrel. Garrison seems to be using the longest barrel, too. So it's easy to speculate that this would be the most penetrating option available, and then to continue speculating that this is the laser Tyler was so agitated about.
When Pike breaks out of his jail cell, he first uses the long barrel in the top (firing) position, then the medium one. This is probably in keeping with the elevator scene: he'd first use the most penetrating beam (which would blast a clean hole into the cell wall), then seeing how this "does not work" he'd select the second-best alternative which might be the realm-phasing beam (blasting the large irregular hole in the wall).
To threaten the Talosian in the cell, he apparently continues to use the medium barrel, which thus probably isn't stun... The prop has twisted a bit more, tho, so perhaps he's actually using the shortest barrel here? Up on the surface, the gun remains in that position, with the two shorter barrels competing for the topmost position (which we know is the firing position, since that was consistently where the beams emerged from in the elevator-shooting scene, on all rotation settings).
So there's surprising (if probably mostly accidental) consistency to how the gun is used - consistency that makes me dare speculate that the long barrel is the laser, the short one is the stunner, and the medium one is the phaser. According to this Swiss Army Knife theory, that is.
Continuing with the fun, "Where No Man" uses the slightly modified prop so that when Spock guards Mitchell at sickbay, the shortest barrel is up. Valid for stun... When he threatens Mitchell with the gun down on the planet, the same situation obtains.
It then breaks down a little. In "Man Trap" we see the shortest barrel deliver whitish beams that collapse a rock column. We could well assume consistency here, though: Dr. Carter would no doubt try to use the stun setting against our heroes, as he is not a bad guy at heart. Even stun could topple the old relics, though (and I don't mean Kirk or Spock). Although we might also acknowledge that Carter could have used a blasting setting first, then turned his gun to stun after forcing our heroes into a corner. We only see Carter's setting close up after he has ceased firing the landscape-modifying beams, after all.
In "What Are Little Girls Made Of?", we get our first real deviation, as it is the long barrel that completely vaporizes the copy of Kirk, and later also Dr. Korby and his favorite sexbot. The medium one should do that if it's the putative phaser barrel...
Timo Saloniemi