Or simply code that after eg 4-7 shots the reload speed goes down a bit every 4-7 shots due to corrosive gun powder
Why. How would that add to the gameplay? That's not even realistic. People can shoot many more times than that with their musket before reloading becomes difficult from barrel fouling. Muskets which became difficult to reload after just 7 shots probably would not be used. And if the effect which you suggest does not change reload speed significantly, what do you intend the change to achieve?
All of you. THINK. Think for a long time about what you are trying to achieve by asking for any change.
It's not 'Lol let's just make changes, because.'
It's: 'We have a specific style of gameplay we want to create, let's make the following changes to bring it about.'
The developers will be willing to listen, and more likely to make a good game if you know what you want out of the game.
One misfire out of 100 shots is a nice, simple balanced number. It will occur commonly enough to not be totally negligible, and rarely enough to not make muskets unreliable. Misfires allow for some realism, as well as further reliance upon melee weapons.
Personally though, I do not care for whether it is included or not, as long as it is not as common as say, one in ten shots.
There are more important matters to think about.