Custom Missions

The only custom mission format for this sort of thing I can speak to is the ZK mission editor. In which case:

• In general, multiplayer missions are currently unsupported. Sorry.
• The minimalist distribution approach is to simply distribute the mission archive (.sdz format) with its startscript (.txt format) by uploading them somewhere for download, and tell users to open the startscript with spring.exe. But this is functionally singleplayer only, requires a bit of extra setup on the user's end (editing springsettings.cfg) and is pretty ugly in general.
• With a bit more work, you could in principle put the mission on the campaign screen and distribute the modified campaign files.
• This still doesn't support non-commsharing multiplayer, although game inviting/joining is automated at least.
• What we'd really want to do here is give them their own campaign so the base game's planets don't get in the way, but changing campaigns isn't supported either.

On that note, recently when I've tried to play the silly maps, functionality seems broken. Last time I played duke nukum the ultimatums did not shoot/still existed after they died and Finnish flag did not rain. as for the finnish flag, I played version 1 which I don't know if it has the rain, but the latest one didn't want to download/load.