It appears that the US Army is looking for a subcompact personal weapon system for people like tankers and truck drivers.

Seeing as how this is a niche weapon, there are basically two cartridges taht make sense, 5.56 NATO, and 9mm parabellum (used in the Baretta pistol).

I would recommend the former, because if you are carrying more than a 50-100 or so, you have less weight, and even out of a short barrel, you have longer range.

I think that a Bullpup makes sense, because you start getting to absurdly short barrel lengths for the weapon (<10 in).

So, you have existing weapons like the MTAR-21 Tavor, the AUG carbine, etc,

If you want to go to a different round, then something like the FN P90, with its 5.7x 28mm cartridge (as compared to the 5.56x45mm NATO) makes more sense than a 9mm round.

Note that all the Bullpups have integral optical sites, which for someone with little experience shooting a personal in a combat situation is a real advantage.

Additionally one of the arguments against a Bullpup, that slinging a grenade launcher under the barrel become problematic, is not an issue here.

If the army is serious, they would buy something off the shelf, but I expect that there will be instead some sort of new weapon developed, so that some General can get a cushy job on retirement.