https://mods.factorio.com/mod/bullet-trails
Managed to narrow it down to a conflict with this mod by disabling everything else. I'll attempt to report this to that mod's author as well, but they're restricting reports to Discord. (Erm, not sure what markdown this uses, but the bold text is supposed to have double underscores (2x _) on either side of it....)

Failed to load mods: bullet-trails/data-updates.lua:14: attempt to index field '?' (a nil value)
stack traceback:
bullet-trails/data-updates.lua:14: in function 'add_trail_to_ammo'
bullet-trails/data-updates.lua:23: in main chunk