It's not pushing the tech "a bit too far". It is simply taking the tech were it has not gone yet, not because it is impossible, but because there really has not been a need to go that route. The statement " you are never going to get something the size of a pencil lead to do anywhere near as much damage as even a 9mm" sounds a lot like (paraphrasing, and knowing the "telephone effect" of the actual quotes) "the car will never replace the horse"
You are taking my statement out of context...
I mist certainly never said ti would be impossible to make a pencil lead as dangerous as a bullet... I said it would never be more
cost effective to do so, because the same explosive tech that is used make your pencil lead sized dart explode, could be used to fill in the hollow point of a bullet, which would be an even bigger bang, which would have better range and accuracy.
For what you want to happen, its not just 1 technological leap that would be required, its several... in different directions.
First you have to create an explosive that would be effective in that amount, then you have to make it stable enough so that the action of firing it would not cause it to explode, but volatile enough that it will still explode on impact. Then you have to create a system that will fire "the needle", and fire it fast enough that it's near weightlessness will not cause it to lose velocity and have a range comparable to a handgun. Then you have to figure out a way to make the needle accurate. Then you have to figure out a way to get the needles to load properly because depending on what method you use make the needle accurate and stable, could have great impact on how the needle would have to be loaded, especially in terms of ammo capacity.
And you have to do all this for a cost that would be relatively comparable to a good old fashioned slug thrower.
And that's not even getting into the legal ramifications, or the fact that any spare ammunition you carry will pretty much be nothing but a a few dozen (I am being generous here, as you did not state an ammo capacity for these things) explosives, each with a destructive force comparable to the damage done by at least a 9mm bullet, designed to detonate on impact, which means in essence, every spare mag you carry is pretty much a pipe bomb waiting for a stray round to hit it, or maybe a taser, or maybe just falling to hard...
Possible does not mean feasible. And it certainly does not mean cost effective.
Caseless ammo is possible, we have working version of it... it is just in no way shape or form cost effective. Gyrojet weapons are possible, they just aren't cost effective, or practical.
As I said, your weapon is cool, but its more Star Wars, or maybe Star Trek, than Cyberpunk.