You also might consider loading an older brass-based 28 ga. round entirely with lead shot, sealing it with a glue gun to be certain, and placing it atop your magazine spring, followed by the tube's end cap. I did this for years with a Winchester Model 12 Trap gun and upon disassembly, everything was intact.....no rust, no spilled shot, etc. Diameter of your specific shell fits the magazine diameter than most weights you will find, and remains invisible as opposed to external weights.
Rob
Last edited by Robt. Harris; 08/09/16 12:20 PM.