Found it. It is on page 16 of the 11-87 instruction manual. It is the part that snaps into the ring that houses the gas ports on the underside of the barrel. I believe the spring expands to meter off pressure from higher power loads. The slug barrel may not have one as it was assumed that one would only shoot slugs from it.
http://pdf.textfiles.com/manuals/FIR..._1100-1187.pdf