Actors were infinitely tall for the purposes of whether two objects could pass over each other, but certainly not for aiming on the z-axis. Think about it; how else could they pass through areas with different z-heights? (i.e. a lost soul floating its way up to a window)
To make it obvious, imagine you're standing on a ledge in Doom and many enemies are crowded right beneath it. If you fire your chaingun off, it certainly doesn't hit every enemy in the LoS, even if they're 500 units below. Gameplay would have been a lot more lame if it worked that way.
Having said all that, it's only really relevant when there are different floor heights. In an area where everything is on the same level, hitting the barrel is basically a guarantee. Hence it's basically irrelevant for DoomRL.