I would like to find a 18" or 20" barrel with ghost rings and hold a tight group.  I have read about some but I can't remmember who made them? Thanks for any help

I sell [RIOT BARRELS] factory remington 870 18inch barrels that will hold 3 inch magnum rounds and 2 3/4 shells. they are bead sight only.in very good condition $75 shipped. call me for more info. JASON IN VA 757 715-4689

ghost ring on the barrel?  that's new to me.  

usually they are mounted on the receiver.
i have seen a cheesy looking fiber optic "turkey hunting" ghost ring.  i guess you could mount it on a 18" barrel.

Like some of the others said, Ghost ring sights usually consist of a rifle-style post on the end of the barrel and the ghost-ring rear on the receiver itself a few inches in front of your eye.  Most reputable gunsmiths can install the rear sight for you at a modest cost.  Wilson combat and many others sell the parts needed for this.

Barrels with regular rifle-style front sights are easy to find.  I got mine at www.cdnninvestments.com.

If you do not want to tap your receiver for the rear sight, get a non-rifled barrel that has rifle sights.  This has a shorter sight radious, but is still useful and can get you on target very quickly.  Rifled barrels spread out buckshot too fast, so stick with smooth-bore barrels.  Regular Foster slugs are typically accurate out to 100 yards easily in smooth barrels.

Here is a good one rifle sight barrel that is non-rifled:

http://www.midwayusa.com/rewriteaproduct/381172

Hope this helps!