Here is a pictoral step by step instruction for the MI rails ( how to cut the delta ring without marring up the barrel )
BTW - the guy that posted this is "Stickman" who takes some of the best AR photos on the web. You see lots of his pictures on Arfcom
http://www.weaponevolution.com/forum/showthread.php?t=3
As already posted, the "best" way is to remove the barrel but unless you have the proper upper vise block and barrel nut wrench and punches for the FSB then dremel away. This is the big selling feature of the MI rail and other 2 piece is you don't have to remove the barrel.