Congrats on your purchase.
I've only ever bought 1 new gun, over a counter. It's a Mossberg 500 and, still my only "real" shotty! I saved for it (had it on layaway before I actually had the FAC) for months...I was 15 and, have loved it about 25 years now.
The nut can be loosened with a tool (be carfeul not to mar it...) and will loosen up a bit over time. For reassembly, you'll feel the barrel "lock" into place and, snug the nut up tight by hand.