Originally Posted By: Kutter

If it's a 28ga chamber and bore, you can follow this link and scroll down to page 24.
It'll give you the specs for both the chamber & the bore for the 28ga. See what your's is in comparison.
http://www.saami.org/specifications_and_information/publications/download/209.pdf


At around .626 base for 28 gauge modern plastic, that leaves around .040 of slop. Maybe the 28 gauge could be fireformed to expand that much.