Well, it's carbine length, which cuts out any of the 20" models and the barrel has the cut outs for attaching grenade launcher/underbarrel shotgun, so it's most likely an M&P 15, M&P 15X, or M&P OR (and probably the OR since you didn't mention removing the A2 front sight) all of which have a chrome lined barrel - your's is probably chrome lined.

contacting smith and wesson directly or searching around a little should help you with the age of your gun.

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686/M&P9 Pro/M&P15X/M&P15-22