It depends what that duty is, and on the "AR-9" in question.
The major limitation of the 9mm round is against body armor, but my 16" 9mm AR will launch 115 gr bullets at 1500 fps with a standard pressure load, which is on the borderline for what Level IIIa body armor can withstand. I can get 1600 fps out of it with a +P load but even with a 10 oz buffer it's moving the bolt faster than is healthy for the carbine.
There both Glock and Colt pattern 9mm AR lowers. The Glock use Glock 9mm magazines and that can be a plus if you shot a 9mm Glock pistol.
However I have yet to see a Glock pattern AR-15 based 9mm that has a bolt hold open that actually works, so if you want to use Glock magazines in a 9mm carbine, you need to look at options other than the AR.
The Colt pattern magazines are Uzi based and Uzi magazines will work fine in a Colt pattern 9mm AR - you just have to mill a cut for the bolt stop in them. That's harder than it sounds as the steel in the Uzi mags is hard and I had to spot anneal to get the bit to cut easily. The Uzi mag is on the left, and a Metalform made colt pattern magazine is on the right.
The Uzi mags are stamped, folded and then spot welded along the back where the metal overlaps. The Colt pattern mags are extruded and there is no over lap in the back. That creates space inside at the back of the magazine for a tab to activate a bolt stop to hold the bolt open after the last round is fired. Thus the Uzi mags will not activate a bolt hold open.
Uzi mags were available up until recently for $10-$12 in new condition so it was worth converting them, given that a new Metalform made Colt pattern magazine will cost between $28 and $40 depending on who is selling it, and if they are on sale. But the Metalform magazines are very well made, very reliable and very durable.
Not all AR9s are created equal either. Colt pattern lowers come in both dedicated lower and magazine block lower formats, and there are three major different types of magazine blocks. A decent magazine block will run you around $150-$200, so unless you have an AR-15 lower you want to use it's cheaper to just get a dedicated lower, and if you want to get a Glock patter lower, a dedicated lower is your option.
I have two. The first was built off a PSA AR-9 16" carbine, although to be honest the only thing that is still on it is the lower as everything else got replaced. It just would not shoot to the level of accuracy I wanted. However with a RRA upper and a free floated Ballistic Advantage barrel it's now a solid 2 MOA carbine at 100 yards with 115 gr HAPs of XTPs.
My other is a 10" RRA LAR-9 pistol with a brace. It's equally accurate with the same loads and only loses about 50 fps in velocity. It also has a 11.25" barrel given that the barrel is a bit longer in the blow back operated AR as the barrel comes all the way back to the bolt face, which is located at the front of what would be the bolt carrier in a .223 AR-15, since no barrel extension is used in the blow back operated 9mm AR.
It's critical that the bolt and buffer assembly be heavy enough to manage the load you are shooting in it. If there is not enough weight in the system, bolt velocity will be excessive and you'll start breaking bolt stops in a Colt pattern AR-9.
I zero mine at 125 yards with the 115 gr HAP or XTP at 1500 fps, which leaves it 3" high at 75 yards, and 4" low at 150 yards. At 150 yards it still has about 1021 fps left, compared to 1130 fps at 100 yards.
So if the "duty" in question requires 2 MOA accuracy at 100 yards and an effective range of no more than 150 yards, then it'll work. The noise level is significantly lower than with a 16" AR-15. The 9mm will produce about 117 dB in a 16" carbine, compared to about 156 dB for a 16" .223. That's a big plus for the 9mm if you ever have to shoot in indoors in an emergency.
Typical 100 yard group:
