This is actually very logical, but quite difficult to explain.
L numbers are allocated to a store and are independently allocated to each type of store. Just as there was a .303 Ball Mark VII and a .303 Tracer Mark VII, there is a 5.56 L18A1 and a 9mm L18A1.
The nomenclature is based on the hierarchy of the description, ie. "Round 9mm Ball.....L18A1" differentiates it from "Cartridge 5.56mm Blank....L18A1".
Note also that there is the difference between the use of "Round" and "Cartridge". A cartridge with a bullet is a "Round" whilst one without a bullet is a "cartridge". In the British 7.62mm series there is the Grenade Launcher L1A1 and L1A2, but there is also the Drill L1A1 and L1A2. It has frequently been published in the past that this is an error in the allocation of L numbers, but it is not.
The Drill L1 has a bullet so it is "Round 7.62mm Drill L1A1" but the genade blank does not so it is "Cartridge 7.62mm......L1A1".
Also, there is no correlation between the L numbers allocated to different types of store. Thus for the 7.62mm the L10A1 is a blank, for the .50 BAT the L10A1 is a drill round.
I hope that is as clear as mud!
Regards
TonyE