Actually, reading it now, the line
The weapon’s measurements
and weight are comparable to that of assault cannons, and while its
damage potential is slightly below that of an assault cannon round,
the Thunderstruck’s rate of fire is higher.
suggests that it doesn't fire assault cannon rounds. It's just a super-high-velocity slug thrower.
So, when it refers to ammunition, there's noting there to make you think they aren't talking about regular projectiles while there is something which suggests that they aren't talking about assault cannon rounds.
...in which case, you could load APDS and such in a Gauss rifle, cementing its place as the most ridiculous weapon in the game.
