You have to change the entering players interior to the listed universe id, and set his position to the given x/y/z to teleport him into an ammunation store.
You can do this for example with pickups. Create a pickup in front of a door that should lead to the store and save its pickup id. Then, in OnPlayerPickUpPickup, check if the picked up id is the id of your "enter pickup". If yes set interior and position.
For exiting it works the other way round. Create a pickup at the store exit, and set the player back to the "normal world entrance" when he picks it up.