Wel, we all know Rumble Balls were invented to "creat waves to disturb frequencies emitted by Evil Fruit"
That means Rumble Ball can be used for other DF users, not Chopper alone, right? Do you think in the future, Luffy will use Rumble Balls to get to gear 2 - gear 3? It does say that Rumble Balls can only last for 3 min though.
Do you think Chopper will ever sell Rumble Balls for SH travelling fee? :D That's gotta be some hot stuff on the market.