Riot leave Gangplank Alone!
First you guys come up with an awesome backstory for Gangplank, then you kill him for a week, then a few days later riot kills him again for another week due to a bug, and now you're planning on killing him again through weakening his most influential part of his kit. At this rate you're going to have to allow gangplank to stack dead man's plate because you're loping off his other arm.
Of all the reworked champions, Gangplank is arguably going to be the worst one of them all in this inevitable bruiser/assassin meta, especially when mordekaiser's winrate has been climbing once everyone started to understand how to play him in this new meta (hope your other lanes win so you can catch up via becoming a dragon-bot and then 1-shot the carries when your gold advantage is high enough through securing objectives).
The only champions Gangplank's barrels pose a threat to are also the champions who have the easiest time avoiding it via destroying the barrel or using one of 20-30 league of mobility abilities. Ever try to set up some barrels against their adc/duelist type of champion? By the time your parley is in mid flight the barrel is destroyed, effectively wasting two abilities at once. Want to try saving cooldowns by meleeing the barrel? Prepare to get obliterated because Gangplank isn't tanky enough baseline to make any use of any armor items (especially after their nerfs).
After these nerfs what is Gangplank left with? Barrels that are only ever useful against adcs who can hit them faster than you can, so it's become a high risk-high reward to a high risk-medium(low) reward mechanic (similar to khazix's state, where he's only worthwhile to play when specific and unlikely situations come up). So we're going to have to go back to pre-rework Gangplank gameplay-wise because our E is going to be not worth using in all but the rarest of occasions.
Not to mention the most dangerous part of an explosion is the concussive force of the shockwave, which goes right through heavy plating.
Gangplank isn't that popular compared to champions like Fiora/Darius/Skarner/Garen/etc who have all been reworked in some way that makes them superior to their former selves (darius's Q now has counterplay, but now he can pentakill the entire team if he manages to kill someone with his ult due to max damage as opposed to having to build it up) But yeah, let's nerf the only one who's remotely close to being balanced so everyone can become flashy LCS players.
Riot please reconsider, only enemy teams oblivious to game mechanics are even affected by his barrels now, you're just giving them more room to make mistakes.
EDIT: Fiora's rework which came out at around the same time hasn't seen nerfs of this magnitude despite being infinitely more useful/durable/dangerous than Gangplank.
EDIT: origen played Gankplank and he went 3/3/3 and their team lost, yet people say he's OP when a player who is arguably better than all of us struggle to keep a positive kda despite his "safe" early game
Gets 60% Armorpen on a fairly low cd ability. Considered to be ruined by a reduction to 45% Armor pen.
Gets only 50% Armorpen on an Ultimate ability. Considered free stats, and dubbed the Feelo Samurai.