Dave, if you're referring to the the space between the breech end of the barrel and the standing breech of the frame, there should be no visible gap there when the action is locked up, a .001" to .002" feeler gauge blade placed between the the barrel and frame near the top and to the left or right of the firing pin, with the action closed on it, should be hard to pull out if not impossible. If yours has a visible gap and the latch is definately fully engaged, contact H&R and ask for them to fix it. If it's relatively new, they'll pick up the tab for shipping if you ask!!
Tim