I removed the handguards on my AR-15 and found that there is a cylindrical piece of metal which surrounds the barrel. It is about 1.5 inches in diameter.  It doesn't appear on this site's parts list for the AR-15. What is this part and does it need to be there? If it doesn't need to be there? How do I remove it? I thing it makes the rifle unnecessarily heavy. Can any one explain this?

Motorsoft

does it run the length of the barrel under the handguard? sounds like a free floating system, used for competitive shooting

ETA: look here

Yes. it runs the length of the barrel under the handguards. What does it do? Do I need it? How do I remove it?

Motorsoft

UNless your shooting CMP matches our the rifle under sling tension in the prone pos, you dont need it. It can make a solid mounting point for a bi-pod.

All depends on what you want to use the rifle for.

you have to remove the FSB and fs or muzzel brake. then unscrew it. you may need to add some parts. Others will know more.