Hello, and thank you for the help!
The overriding of createSpawnPacket didn't seem to work, though thank you for catching it before it didn't work in multiplayer!
Although I'm using the constructor that actually takes in a shooter (I'm having another item spawn the projectile using it), Forge seems to try to spawn it using the FMLPlayMessages.SpawnEntity spawnEntity, World world constructor, where I can't seem to find the shooter, leading me to believe it had been lost. Would there be any way to recover it, and transfer any other data along with it?