Warp coil rail guns maybe--not unlike what accelerated the Warp 9 probe Worf's lady used....Usually so powerful only starbases can firethem.
But Vengeance is a flying starbase.
Is it possible that this is is the basis of all photon tech? With the being an uncharged warp coil, and the photon launcher "charging" that casing with a warp field to fire it? It makes more sense than the photon launcher just being magnetic rail excelerator, especially given k'ehleyr was able to fit in a probe/torpedo casing. There is no way she would fit in it if there had been some kind of warp propulsion inside the casing.