I wonder if it would be practical to have the soldiers all armed with a pistol, maybe the new FN high velocity for use in close quarters as a primary weapon instead of the M4. For that matter a high capacity .45 would be good.
For those times when they really needed more power they could use an FN FAL or something similar. In other words, every combat soldier has a pistol and a rifle. The pistol for close quarters and the rifle for everything else.
The old tommy gun was awesome.
.40 or .357 Sig would be better. More expensive but better. MP5 comes in 40 cal and works very well. Adapt the M4 for .40.