[Charles Martel]

To me it’s like firing a .357 magnum cartridge through a 1-1/2 long barrel. All you’re going to get is a horrendous muzzle flash while achieving the muzzle velocity of a +P .38 Spl. Maybe.

There are some pistol caliber carbines where that is a concern - CMMG has a 10mm version, IIRC. However, most of these 9mm ARs have barrels at least as long as would ordinarily be found on a Glock, so I don't see where wasted gunpowder would be an issue.

People generally shoulder these things PDW-style, and since it's legally a pistol, no worries about having an unregistered SBR.

[Who is John Galt?]

There are some pistol caliber carbines where that is a concern - CMMG has a 10mm version, IIRC. However, most of these 9mm ARs have barrels at least as long as would ordinarily be found on a Glock, so I don't see where wasted gunpowder would be an issue.

Bingo. An AR pistol, chambered for a pistol cartridge, can actually improve ballistic performance, if the barrel is an appropriate length (8"-12"). Other cartridges, like the .300 Blackout, may also function efficiently in barrels less than 16" long. A lot of folks apparently picture .223/5.56 when they hear "AR pistol", and there's no doubt that the performance of that cartridge suffers, even in ~14" M4 barrels.