1. Create a projectiles on 1.12.2

#(minimum is id:data. Named items id:data:displayName:lore1:lore2:....)
  loadingItem: '397:3:Cannonball'

2. Try the same with 1.15.2

#(minimum is id:data. Named items id:data:displayName:lore1:lore2:....)
  loadingItem: 'player_head:-1:Cannonball'

1.15 doesn't have data anymore so it's not working.
Regular player_heads work fine but adding a name check to it doesn't i tried

#(minimum is id:data. Named items id:data:displayName:lore1:lore2:....)
  loadingItem: 'player_head:-1:Cannonball'
  loadingItem: 'player_head::Cannonball'
  loadingItem: 'player_head:Cannonball'
  loadingItem: 'minecraft:player_head:-1:Cannonball'
  loadingItem: 'minecraft:player_head::Cannonball'
  loadingItem: 'minecraft:player_head:Cannonball'

I don't know what to do to get named projectiles to work on 1.15 now.

I had to do some testing and it turns out Minecraft added a $r in the beginning of the tag.
For separation of the id and display name use ; instead of :.
The example below works for me.

#can be loaded with stone and it will fire an explosive projectile (minimum is minecraft:material. Named items minecraft:material;displayName;lore1;lore2:....)
#https://www.digminecraft.com/lists/item_id_list_pc.php
loadingItem: 'minecraft:cobblestone;§rCannonball'

I tried what you said above

#can be loaded with stone and it will fire an explosive projectile (minimum is minecraft:material. Named items minecraft:material;displayName;lore1;lore2:....)
#https://www.digminecraft.com/lists/item_id_list_pc.php
loadingItem: 'minecraft:cobblestone;§rCannonball'

and it doesn't work for me. I tried getting the item via creative and renaming in an anvil and it doesn't work. I tried getting it through essentials with /give Wolfebersahd cobblestone 1 name:Cannonball I tried with playerheads too minecraft:player_head;§rCannonball with no luck. Even when i use just minecraft:cobblestone only the vanilla un-named one works any other cobblestone doesn't work at all. I can also use the un-named minecraft:player_head but adding a name to it prevents it from working for me completely.

I'm using:

• Paper-284 for MC 1.15.2
• Latest cannon on bukkit: Cannon-v2.5.6
  

I used
/give @p cobblestone{display:{Name:"[{\"text\":\"Cannonsball\",\"italic\":false}]"}} 1
generated by https://www.gamergeeks.nz/apps/minecraft/give-command-generator

With this the §r is not necessary.

I'll try that right away.

That seems to have worked... thanks... i'll play with that a bit more since i use custom player heads.

so far

  loadingItem: 'minecraft:player_head;Cannonball'

works fine!!!!