For users in 4.18, after swapping everything to EditDefaultsOnly you won’t be able to just compile the BP. You will need to delete and redrag out the Turret and Barrel pins for the Set Barrell/Turret reference in the event graph.
At this point everything seems to compile but fatal crashes happen on play until you go back into the BP and set the Projectile Blueprint again as it ends up reset to nothing with the defaults change.