ZDoom

Understandable! Odds are good you'd never be firing at enemies at such an extreme angle anyway.

The horizontal spread is entirely calculated in 2D, so for such extreme angles the horizontal disance is very small and as a result the spread very low. You'll find similar behavior in other areas, too. Some have been fixed, like projectile speed but for others it's not so easy. Here you'd have to do a complete overhaul of the trace calculation. Not going to happen.

At extreme view angles, weapons cease to have horizontal spread. I've only tested this with hitscans so far, but it wouldn't surprise me if projectiles had this problem too.

It's very easy to recreate: just give yourself a hitscan weapon, type either noclip2 or fly in the console, go to the top of a tall room, then fire directly at the ground. I even have screenshots illustrating this:
[spoiler][b]Normal spread when shooting at a wall:[/b]
[img]http://i.imgur.com/oaUegmj.png[/img]

[b]No spread when shooting directly at the floor:[/b]
[img]http://i.imgur.com/ux7UoOx.png[/img][/spoiler]

I have no idea what version of GZDoom this started in, but it exists in the [url=http://devbuilds.drdteam.org/gzdoom/gzdoom-g2.1.pre-508-ga8398e7.7z]latest build (2.1.pre-508-ga8398e7)[/url] I've tried from DRDTeam. It may not even be a bug given limitations with the Doom engine, but this doesn't seem right to me so I'm posting it anyway just in case.