If the barrel is loose when the action is closed without the forend installed, it needs to go back to H&R for replacement as they won't shim it to repair it, but you or a gunsmith could, see barrel fitting in the FAQs and Help sticky on that.
Shimming the forend spacer as shown in the FAQs will tighten the barrel when the action is open, but it will also mask a loose fitting barrel which will still be inaccurate.
Tim