Just saw this, time to chime in.
L-Mark-Two shells are designed for anti-emplacement and anti-starship usage, and are intended to allow the L-Mark-Two to perform comparably to the anti-starship turrets used by Star Army of Yamatai vessels. They are a far more tame option compared to the 2,000KG compressed positron shells used by Sakura-Class and Plumeria-Class gunships.
https://wiki.stararmy.com/doku.php?id=stararmy:weapons:ke-s3-w2901_positron_accelerator_cannons and the Striker array found on Yamataian ships
https://wiki.stararmy.com/doku.php?id=stararmy:striker_array.
By this reasoning, the 9kg of antimatter material found in a L-Mark-Two slug is comparably tame for the usage, with damage potential largely being linked to acceleration and blast direction. While the damage is substantial, draw-backs originate from ammo capacity and speed limitation.
EDIT: Further note, the SAoY already has antimatter shells in its inventory, those used in their shoulder cannons. M'Cel launchers are designed to be compatible to those shells with the use of an easy to insert modification to the barrel, as indicated in the wiki article.
EDIT2: Also forgot to mention, the original ammunition manifest was written back when we were only stressing the diameter of a round, as opposed to also including the length. It should be noted that the L-Mark-Two round was intended to be longer than grenade-launcher style 40mm, for the L-Mark-Two its meant to be the dimensions intended for 40mm artillery pieces, with a length of somewhere in the ballpark of 300-400mm
EDIT3: Also, an image to serve as a stand-in for our slug, I bring you employee of the month, inanimate carbon rod.