I've used them in giving sporting clays lessons. They help to demonstrate where the shot is going for someone who can't seem to get the lead right. Kind of pricey, though.
Same type used in WW2 to train bomber air-gunners near here.
BB
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If we shooting sportspersons don't hang together... we will all hang separately !
Never knock another's different shooting interest or discipline...REMEMBER we are all but leaves on the same tree of freedom.