This is the checker for the 7 shot Model 986 Revolver chambered in 9MM.
The cartridge openings were cut using a chamber reamer to a minimum dimension.
The openings are tapered and are an exact fit for the 9x19 cartridge.
This is used to check your moon clip without putting live ammunition into the chamber.
If the loaded moon clip fits in this checker, it will fit in your gun!