My wifes uncle just purchased a Rem 11-87 and the ownersmanual discusses the use of abarrel seal activatorfor 2 and 3/4 inch shells that should be removed for 3 and 3.5 inch shells. This might only be for Remington models, but just thought this may have been what rcaddell heardpeopletalking about.
Page 15 in the pdf file
http://www.remington.com/pdfs/om/om_11001187.pdf