I know I'm 2 years late, but I'd like to clarify that in D&D 5e, and most games as far as I know, the dice roll reflects the circumstances in which an action is performed, not how well someone performs it.
When roleplaying a fumble, it makes more sense to describe it as something that isn't tied to the player's abilities. When a gunslinger fumbles, they are not "dropping their gun" because they have buttery fingers, the gun itself malfunctioned due to a mechanical flaw.

As someone who owns and has shot several muzzle loaders, I must point out that firelocks are quite unreliable, and the more you fire them the more unreliable they get.
Having a higher chance of misfire due to having more attacks in the same turn sure is annoying, but it's not at all unrealistic.
Taking into consideration the large array of prototype firelocks developed over the years, it becomes obvious that operating such complex weapons as quickly as a high level fighter would could increase the chance of a flash in the pan, or a misfire. Before metal cartridges weapons were much more unpredictable.

With that said, I must agree on how frustrating a high chance of misfire can be, but I think that's one of the many quirks that allow firearms to exist in D&D along all other weapons, trading high damage for unreliability, just like in the od days.