There is no real basis for a case of
RAW or
RAI being available for
SB carrying models getting/using the ammo. Codex Space Marines, Page 99: "A storm bolter resembles two bolters attached side by side." Mat Ward did not say it IS two bolters attached together, he said they only resemble that, therefore
SB are not bolters/boltguns. Only boltgun carrying models are given the special ammo, so no ammo is available for the
SB carrying model to use. If
GW wanted the
SB to get, much less use, the special ammo, they would have spelled it out as they did with the bolters that are part of combi-weapons. Codex Space Marines, Page 97: "Combi-Weapons are bolters that have been specially modified...".
At least a bolter/combi-weapon carrying
IC that has joined the Sternguards has a case (a very small one at that) at getting ammo based on
RAW since he would be a "bolter-armed model in a Sternguard Veteran Squad", the model therefore "automatically comes with several special ammunition types."

I am not going to argue for this one but there is at least a case for it, however small.
If someone somehow gets a
SB that is allowed the special ammo, it would have its profile replaced by whatever ammo is being used at the moment. Sorry, no Assault 2 with the rest of the special ammo profile being unchanged for Sternguards.

Maybe they can get psycannon ammo next codex, since it can be fitted in a
SB.