I've shot steel gongs extensively and if you make the gong to big and heavy the inertia is to great, thus every round you fire will cause more damage I've found that if you make a 1/2 inch plate less than 40 cm diameter and you hang it at a slight angle(allowing it to face downwards) the bullets ri cochet into the ground thus exerting less energy in your gong and prolongin g its lifetime,
OR ask a friend on a farm or at a scrap yard for a Disc implements ,old dis cs and weld something to cover the hole in the middle and you have a higher carbon steel gong already nice and round ready to shoot and cheaper than s heet metal. I also found that they outlast the mild steel sheet metal gongs by far. And on low velocity cartridges ricochets are not dangerous I have been hit a few times and I would compare the worst to bee sting just be sho re to wear eye protection, but if you suspend your gongs at the right angle s downward the chance of ricochets coming back at you are very small.
Jan Griesel