it is emitted from the lower dome
Well, it is "fired" from a "tube".
Plus, the torpedoes being fired, or possibly the tubes involved in the firing, are numbered - 2, 4 and 6 are fired in "Journey to Babel" while 2 is selected in "The Changeling".
If we're talking about wrapped energy, then giving numbers to the torpedoes makes little sense: they will only come to existence at the moment of firing, gaining their number no earlier than that, and so there won't be an obvious way of firing 2, 4 and 6 while leaving 1, 3 and 5 unfired. On the other hand, if these are tube numbers rather than projectile numbers (as submarine analogies would suggest, and better explaining Kirk's odd desire to skip odd numbers), then it doesn't matter whether the projectiles being fired are physical bullets or energy wraps.