OK, I'm going to go out on a limb here and say...maybe the sidearm shouldn't be a pistol. No, I'm not suggesting the Serbu Super Shorty shotgun -- attention-getting but only three shots.
If we are stuck with ball ammo, and likely 9mm, maybe the sidearm needs be a pistol-carbine? I don't mean a pistol-caliber longarm, but a holsterable, pistol-type weapon with a folding stock that can be fired offhand as a pistol but, better yet, can be fired with a folding stock.
Even if 9mm ball has it's problems, multiple hits will certainly do the job. It's a lot easier for the non-Special Forces types to get multiple hits with a shoulder weapon than with a pistol. Full auto or burst fire might be an option, though ammo wastage might argue for semi-auto only.
There are a lot of examples out there, past and present (Czech Skorpion 9mm, Micro Uzi, even that homely Peruvian assault pistol mentioned in a recent THR thread) but the most likely candidates right now are:
The MP5K is a bit too big, and the Glock arrangement doesn't look like it would stand up to hard use, so my vote is for a B&T MP9 (semi-auto only, or maybe burst, no full auto) to be the new U.S. sidearm.
OK, flame away!