I intended for this to be a feature in my upcoming enchantments datapack but it turned out that was a terrible idea as this is far too unbalanced. Still really fun to play around with though and could be very useful in an adventure map (I don't need credit but it's the nice thing to do). Any crossbow with the enchantment id "llac:auto" will function as such.

If you want the crossbow I used, just run /reload and grab it from the green box.

Happy Spooktober!