[@vancexentan]

Just a clarification: "bullets" fired from Heartbeat don't make noise, because there is no primer or casing as each "bullet" is a crystalized construct of her blood.

The explosion occurs on impact from the trace iron, sulfur, and copper in human blood reacting from the sudden stop. This is accomplished via the alchemical inscription pressed into each "bullet" upon forming in the chamber, as Heartbeat was specifically designed to mirror the formula that lines Heartstring's handle, and weaponize it for range, as the knife uses blood to function like a wasp knife on steroids.

The actual firing process is done by a concentrated, localized burst of mana channeled from the trigger to where the blasting pin would be, and treats the "bullet" as if it were the Gandr spell - as done by Alchemy and not Magecraft.

If Heartbeat has to make any noise, at all, it would make would be akin to a pistol suppressor. For a better understanding of how that sounds, [url=https://youtu.be/ak-QH9x8Hcc?t=34]this video[/url] is timeskipped to a handgun that fires the same caliber of bullet as Scarlett's CZ BD 75 Police Model.

All this to say, Leon wouldn't hear Heartbeat firing at that distance, but he would hear the small explosions that are shattering the streetlamps.