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Updated Answer: You will need a full auto bolt carrier in your firearm for the Echo AR-II to function. I would highly recommend contacting us at techsales@opticsplanet.com to ask about specific proprietary bolt carrier groups for calibers that don't use a mil-spec full auto bolt carrier group.
While this trigger works with any AR-15 full auto bolt carrier, in order for the Echo trigger to work with the AR-9's blow back design, you will need a specially milled bolt in order for the upper to close. I know Stern and American Resistance both sell this bolt. You will also likely need an H2 buffer and AR-10 buffer spring to slow the bolt down enough for reliability.