Nothing a subgun can do that anAR carbine cannot do exponentially better. No subgun can penetrate tactical soft armor let alone any hard armor. And don’t mention that 5.7 round either.
Remember, by definition a subgun fires pistol cartridges.
Subguns can still offer a quieter and much more compact package than a shorty M16 mounted with a suppressor. Any 5.56mm barrel length of less than 10.5 inches is a liability when a suppressor is added.
I’d still have thought the age of the subgun was long ago in the past but for some reason the US Army is ordering more. I hope they do the right thing and just go with the Hk MP5-SLI. Hk has long ago refused to bring back the SD model, sadly.
Every other subgun on that list is unsuitable including the Sig-SAUER and CZ Scorpion. The Hk knockoffs from PTR and Zenith are lousy low-rent substitutes for the original. The B&T MP9 is the rebranded Steyr machine pistol which is high quality and ingenious but the extra trash you need to add to it to make it an effective subgun turns it into a clumsy afterthought. Finally, pistol caliber M16 subguns are just horrid for several important reasons.
Nothing will best the MP5 on this list but I’m counting on the Army procurement process to make the dead wrong choice or abandon the project altogether.
Subguns can be much more effectively suppressed than an AR carbine in 5.56x45.