The machine gun wagon now lives in Military Wagons, together with the machine gun, flamethrower and rocket wagons:

• Mod portal: https://mods.factorio.com/mod/military-wagons
• Source: https://github.com/vertexvaar/factorio-military-wagons

This version is an empty bridge. It contains no wagon of its own and only depends on Military Wagons, so updating pulls in the new mod. Wagons in existing saves keep working.

• Each wagon in Military Wagons can be switched off in the startup settings, if you only want this one.
• When you switch over, the ammo loaded in the wagon's turrets is lost once. The turrets reload from the wagon's cargo or tank by themselves.

Once Military Wagons is installed you can remove this mod.