This idea was passed to us a long time ago, before the new proliferation of accuracy-enhancing services for these rifles was available. Yes, there are now lots of neat things that can be done for these rifles to enhance their potential, but those solutions also entail time and outlay of cash. The idea presented here works! It has cut group sizes nearly in half on many of the guns we have done it to and it requires no gunsmithing services.
The gas block assembly at the forward part of the forend, which is held together with four screws from the underside, is the focus of our attention. Remove the four screws, and then remove the top part of the gas block. Next, remove about .030" of metal from the mating surface of that block by running it over a piece of plate glass or plate iron (a very flat, very stable surface) covered with 320 grit wet/dry sandpaper. By removing the metal from the mating surface you create the necessary gap for tuning, as you will see below.
Now that you have accomplished the removal of approximately .030" from the upper gas block assembly, reassemble the unit with this procedure. When tightening the four screws, use an inch/lb. torque wrench to VERY UNIFORMLY tighten all of them with the same torque, and simultaneously use a feeler gauge to make sure that the gap between the block halves is uniform side to side as they are torqued down.
This little procedure puts pressure on the barrel from the gas block very uniformly, and equalizes that pressure, especially as the barrel heats up from repeated firing. Simple as it sounds, it works! No, it may not be a complete cures-all, but it certainly is a no-cost enhancement.