Aside from shooting a few more high power rounds, the only thing you may try is to remove the stock, then the nut at the end of the recoil tube. The one that is inside the tube. Remove it carefully and remove the recoil spring. It is not necessary to remove the recoil tube! I ran a bronze brush on the end of a drill and polished out the tube. That seemed to have helped reduce the breakin time. If nothing else, I made certain that I removed the heavier oil/grease that the M4 came from the factory with before I lubed it myself. The recoil spring can be changed out to a "lighter" one that handle the lighter loads. Go to the Wolff GunSprings website. If you're shooting at an inside range, that may be the trick. If you were in a situation where you were alternating loads (outside), I would probably just run a few more rounds through it and stick with the heavier spring.