Oh, I forgot to mention how this could potentially be worked into a passive.
Lets say, their passive is something like Jhin's where you essentially have a counter for the number of shots you have out of 10 instead of 4, (and we aren't doing anything about his attack/reload speed or scaling his damage on something ridiculous)
Now, for every 10% critical strike chance, 1 of those 10 shots will be a crit.
They will be loaded as evenly as possible, and the ammo counter could display similarly to how Jhin's shows 3 yellow for 'normal' and 1 red for his crit.
10%
Y Y Y Y Y Y Y Y Y R
20%
Y Y Y Y R Y Y Y Y R
Simple so far, but now 3/10 is 33% which means that if we did Y Y R we would end up with 9 shots. So with this we will load the critical shots toward the end of his ammo counter by placing an additional Y in the front.
30%
Y Y Y R Y Y R Y Y R
40%
Y Y Y R Y R Y R Y R
50%
Y R Y R Y R Y R Y R
Now it gets easy, just fill in the yellows from the back.
60%
Y R Y R Y R Y R R R
70%
Y R Y R Y R R R R R
80%
Y R Y R R R R R R R
90%
Y R R R R R R R R R
100%
R R R R R R R R R R
There are probably other ways to do this, ... probably even simpler ways, but this was just one idea.
I could imagine the ammo counter being kind hard to read with 10 shots tbh, but this would be the basic concept