User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130128042915/@comment-4721065-20130129081220

Well, the codex doesn't really seem to state much regarding the difference between dreadnought & cruiser ammunition, so it's all open to speculation. It might be that the dreadnought round is either larger enough to survive partially and inflict enough damage or the exact opposite, being smaller and more structurally stable, while cruiser round simply has the wrong inertia to survive re-entry. It might also be that dreadnoughts have access to special thermal-resistant rounds for that, but then why wouldn't cruisers? And in either case, the AA gun's projectile would have to be able to survive going through the atmosphere and then still be powerful enough to down cruiser with single shot, which seems to be close dreadnought's firepower without the accompanying size - strange at best.

Perhaps a way to explain this is the fact that some objects can apparently just bounce off the atmosphere without fully entering it, so maybe that is what happens to cruiser rounds due to them lacking whatever it is the dreadnought rounds have and cannot enter at any angle? Since this would never affect the shots fired from inside the atmosphere, it seems to be a plausible solution to the problem.