Exguardi |
Umm. Hire a porter? (Seen here with pistols but you could tweak for muskets easily enough.)
Gilarius |
Beneficial Bandolier will reload one barrel as a swift action each round and the Reloading Hands spell will do another.
Apart from those, rapid reload and alchemical cartridges only get it to a move action. So RAW, muskets aren't usable for multiple attacks per round for anyone except musket masters.