An AR-pattern 9mm carbine? If you really want one, you can buy one. It doesn't have to be S&W branded. The big decision in either buying one or building one is deciding on which pistol magazine you want to use. Most of the ones I've eyeballed utilize Glock mags which are cheap and plentiful.

I've eyeballed 9mm AR-15's but what's the point in one? We are lucky to live in a pistol caliber carbine design rich environment. The Turks and the Zenith clone MP5. CZ and the Scorpion. Sig and the MPX. Kel-Tec Sub2K Gen2. Kriss Vector. The hard to pass up Hi-Point carbine. IMO, buy a pistol caliber carbine and might as well buy a form factor other than an AR-15.

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-John