On my last game, i found out that DMP do not procs on Gangplank barrel's, Is it a bug or just a normal interaction ? I personally pref that it would, but even without it's pretty much k cuz oranges (Btw, DMP obviously proc on Q).
Ps: http://leagueoflegends.wikia.com/wiki/Dead_Man's_Plate if you don't know the item.
Are you hitting with chained barrels or just one barrel? Some effects seem to only work on the first barrel.
Its honestly more usefull that way, because you can now have a 75% slow on your Q and slow them afterwards with Q + barrel
I'm not quite sure that Q actually procs DMP's bonus damage. Don't get me wrong, it certainly should, but with it being a new item with a new mechanic, it could be buggy with GPs Parrrley
barrels don't proc DMB, its working as intended as they aren't really coded as an enemy or minions they don't even count for CS score
Its also more beneficial cause you keep the 60 ms boost
