Hey Bo, could you by any chance be thinking of the VariFlame Primer Adapter? If so, your gun has to be set up to take 209 primers. The VariFlame is the same size and shape as a regular 209 shotgun shell primer, but you load it withsmall rifle or small pistol primers. Check the link.

http://muzzleloadingbullets.com/variflame_primer_adapter.html