Gangplank interactions with his barrels and Lichbane

I might be going crazy but if you buy Lichbane with Gangplank and use the proc to hit barrels you'll be doing less damage to minions unless you just auto attacked them without the proc.

I thought his barrels would deal bonus damage equal to Lichbane's proc but it doesn't deal any bonus damage and consumes the proc.

Is this an intended interaction? It would make AP GP semi decent if you could proc Lichbane on barrels but it still wouldn't be as good as AD GP.