Someone explained it better than I can on TOS. But the bottom line is... it doesn't matter. When you free hand the gong, you get visual and audible feedback on a hit.
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who says the bullet isn't imparting energy to the rigidly mounted plate? the movement is just more quickly dampened by the ground, the mounting stand etc.
also even if it does expend some energy in swinging the gong (which is does) this is not necessarily enough to stop damaging impulse from being imparted to the plate.
if someone punches you in the face and you have a stiff neck vs someone punching you in the face with a wobbly neck... the punch is going to hurt.