Secret Weapon Miniatures are now offering 28mm scale metal shells.
From their announcement:
I am very happy to announce that Secret Weapon has just launched a line of scale shells compatible with several popular weapons.
These shells are available in shotgun, standard, heavy, heavy + and assault sizes.
Each sprue of 10 is priced at $1 so you can get as many shells as you want, in as many styles as you want, without having to order multiple $5 bags and being stuck with extras.
However the cost of metal casting is constantly increasing so this pricing might not last forever. Take advantage of it while you can.