This barrel has not been tested to fit in an RPX9. It is designed to match Glock factory dimensions so is compatible with Glock factory slides or Aftermarket slides that are designed to be Glock compatible.
Sell your RPX9. They are very dangerous with aftermarket barrels. OEM glock GEN 3 will work but there are no threaded or fluted available. If the product says gen 1-5, it will NOT work. The locking lug and lock block DO NOT match. It will lock up your slide while in battery and the only way to get the slide back off is to pull the trigger.