I think you mean 9/64". 9/16 is a huge allen key.
9/64" and 1/8" are very common in firearms from my experiences. Also there is no metric equivalent for 9/64", 3mm is close but doesn't always work. I also recommend a folding key set of allens since I always loose my 9/64" key.
Hope this helps.
9/64" and 1/8" are very common in firearms from my experiences. Also there is no metric equivalent for 9/64", 3mm is close but doesn't always work. I also recommend a folding key set of allens since I always loose my 9/64" key.
Hope this helps.

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