What grade of steel plate have you managed to get your hands on? Just asking as this will determine how long it will last.
My gong is a 250mm square with holes at the top corners. Chain attached with shackles. I have seen a better design where the chain is attached to the back of the plate about 80mm down from the top edge. This gives the plate a slight downward slant, so the bullet splatter heads down, and the chance of a bullet coming back are greatly reduced.
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