There is supposed to be a small gap there. As mnrivrat stated, look inside at the barrel extension/receiver joint; if that is snug, you are set.
DO NOT TIGHTEN THAT SCREW WITH "VISE-GRIPS", PLIERS, ETC. You WILL wreck stuff doing that.
Never mind HOW I know, I just know.
Mossberg has the manuals for the current firearms on their website for download if you need it.
www.Mossberg.com
look under the Customer Service link for "manuals"
I did a copy-n-paste out of the manual, but I couldn't get the pic to copy below...
500®, 505™ and 535™ Models Slide the barrel into the receiver and tighten the takedown screw, finger tight only. DO NOT use pliers or a wrench to tighten the takedown screw. A little space between the magazine tube and the takedown mount is normal.
WARNING(all models): Check to be certain that the barrel is fully seated into the receiver by looking through the ejection port. The barrel extension must rest firmly against the corresponding cut in the top of the receiver as shown in Figure 4.