Downgrade the rifle length and upgrade the caliber
There is much to debate when it comes to discussing the effectiveness of the 5.56 round. When it was originally deployed in the M16A1, the military had issues with the reliability of the gun, but I have heard not one complaint from any Vietnam vet about the lethality of the round. That was when the military was using a 20inch barrel with a 55gr bullet travelling well over 3000fps. The bullet is extremely violent at that velocity. The lethality of the round was greatly diminished when the military started using shorter length barrels and the 62gr SCBT also known as SS109 or M855. Although it is only 6grs heavier, the bullet is so well balanced that it basically zips straight through a target and the terminal ballistics are not very good below 2800fps which is practically what an M4 shoots it at near the muzzle.
In my opinion and many others that have been to Iraq, the short way to get around the probable is to use a heavier bullet. The MK262 is a 77gr bullet that although it does not travel as fast as the 55gr or 62gr, it is much longer and tumbles almost immediately after hitting a target. For close quarters situations I recommend 75gr-77gr HPBT. One other thing to consider that many in the Special Forces or Civillian area are doing is upgrading their M4 or AR15 Uppers to the 6.8 SPC. Do some research on which kind of upper configuration you are looking for because there are a lot of manufacturers out there. The CMMG Upper is a quality upper that will get you in the new realm of M4 Carbine upgrades.