Only partially correct John. The OD of the 4mm adaptor that goes with this muzzle device is 9mm, so the hole is large enough. But without the insert adaptor the alignment is not precise so you are correct that firing a 9mm round through it would most likely be a very unpleasant experience.
At any rate, calling this device a flash hider is ridiculous but Simpson's isn't the only party guilty of this misnomer.
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If it's made after 1918...it's a reproduction
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