An ADC... that can't continuously auto attack? I mean, I don't think it's impossible to make it balanced and interesting, but the whole point of ADCs is to be able to put out damage consistently so either the champion would have to be able to expend their ammo quickly and back off, a bursty assassin or at least more of an AD caster, or the reloads would be frequent enough and fast enough to not really be meaningful gameplay, just an upkeep mechanic that people would tolerate if the character was strong enough.
Something along that theme that I think WOULD be interesting would be like this: Champion still has basic attacks always, but below average in range and attack speed. On top of that, each of the basic abilities would "load" a certain number of special shots, with various effects and varying clip sizes. Maybe one would give improved range, one would give improved attack speed, one would give... I don't know, improved lifesteal? That one might be too OP or completely worthless, most likely OP early (free sustain without needing to build an early lifesteal item) but worthless late when they finally do get an item for it. The abilities would either need REALLY long cooldowns so they don't just have 100% uptime, or (my preference) each ability has a reasonable cooldown, but it doesn't start until all the ammo is used or you switch to a different ammo type.
For an ultimate, there could either be another ammo type that's really good, or something else (that just about covers all the options). My personal idea is that it would give maybe 10 seconds where the basic abilities don't have a limit (so you press one of them and get to use it as much as you want rather than a limited number of shots) and no cooldowns so you could switch freely. I would say that the basic abilities should have a bit of delay involved and an animation, but not actually a cast time so the champion could still move during the animation; the ultimate could also remove that or reduce it. Basically, the ultimate would give that heavy sustained damage that ADCs need, where otherwise you would only have a few attacks of whatever type you need at the time.
The passive I'm not so sure about. I'd probably do something that is just an effect each time an ability is used; it would have to be balanced with the amount of ability spam the ultimate could provide, and I'm not sure WHAT that effect should be, but there might be something there. Alternatively, the champion could be manaless, with the ammo going into the resource bar and the passive mainly just being a description of how it works like Rumble has. Might need to balance a bit differently without a real passive, but I wouldn't really mind that, and we don't have a manaless ADC yet. It gives some extra counterplay because the enemy could easily see how much ammo you have left, but it also means you can spam your abilities as much as you want without worry.
I think that would definitely be interesting in concept at least, though it wouldn't be very flashy especially since the champion would basically have nothing but auto attacks.
? Of course there would be some sort of bonus due to the requirement of managing of the number of autos per clip, but the concept seems really interesting to me. Could have the ult be a 
knockoff that didn't cost extra ammo (not implying 
) and this concept sounds REALLY cool.
kind of uses load and reload mechanics
