What are the benefits of  a 9mm AR15?
And where can I purchase a 9mm AR15 just with a 16 or 14 inch barrel and collasible stock?

What is the shortest barrel i can own without any special requirements?

Quoted: What are the benefits of  a 9mm AR15?

You can use them as a sub-gun in a 3 gun match. That's the only reason I would contemplate one.

ASA has a current sale running on 9mm AR15s for $699.
Never fired one don't know of any pros or cons

Legal length of a rifle barrel without NFA papers is 16" from the muzzle end to the breech face.

Including any permanently attached flash hiders or muzzlebreaks.
Such as:
11" commando carbine with welded 5.5 flash hider.
14.5 M4 with 2" Phantom flash hider.

The down side to this is you can never change out the flash hider to other things like silencers or be able to experiment with muzzle devices.

I have one and like it a lot. I can train on AR15 platform during the week at indoor ranges. Its quieter than .223. If you have your own steel targets its easier on the steel. Box of 100 rounds runs $11.50.  Drawbacks are don't plan on using it out past 100 yards much. Wind plays with 9mm and can blow it around some.

ammo cost.