Most of the economically priced shotgun ammunition made today has a mild steel base plated to look like solid brass. Ammunition with this steel base does not eject as well as that which has the all brass base. I think the reason for this is the steel base does not contract or shrink back to its original dimensions nearly as much as does the brass base after firing. To determine what the base is made of, just put a magnet against the base of the shotgun hull. An all brass base will not be attracted to the magnet. The Peters Blue and Green Magic cases, Remington STS and most of the Winchester AA cases have the all brass base.
SHOOTING THE "COWBOY" DOUBLE SHOTGUN
by Deadly Redly
From The Cowboy Chronicle, June 1990, The Single Action Shooters Society Newsletter