The U.S. Military uses the Knights Armament Corp.’s M4 and M5 rail systems on the M4 and M16 respectively. These rails are good heavy duty products. That said, they are an aging design and there are newer lighter cheaper nicer designs. MSRP on the M4 and M5 RAS run about 320 dollars which is rather high. Factory seconds can often be found for around $150 which is a good deal and far more reasonable.
Installation should be pretty straightforward, yet sometimes people mess it up, so here is a guide.
The RAS has a top and bottom section. On the rear of the top section, there is a single screw. This may be for an allen or a flat screwdriver blade. This needs to be unscrewed allowing a clamp to be released. Newer KAC RAS rails will have a little bracket keeping this screw captured.
On the front of the top rail section there is a little spring tab. This needs to be installed under the front hand guard cap. Earlier versions are flat are were sometimes improperly installed over the front handguard cap, so this spring was changed to be U shaped so it could not be installed incorrectly.
First remove the old clam shell hand guards. The slip ring or delta ring near the upper receiver needs to be pulled back allowing the plastic handguards to be removed. Sometimes the spring will be very stiff and it will help to have another person. There is a tool made for removing and installing these hand guards but unless you do it all the time it is not worth spending the money on it.
Second you need to unscrew the screw on the top rear of the top section of the RAS. Ensure that the clamp is loose and completely hinged down.
Insert the front tab under the front handguard cap, pull back on your rifles slip ring or delta ring, and lower the rear of the RAS in place. Make sure that the clamp is sitting under the barrel nut.
Note: The M5 RAS for the M16 Rifle has an optional accessory piece sometimes used when mounting a M203. If you are using this piece it goes inbetween the front of the top rail section, and the front handguard cap.
Make sure that the clamp on the back rear of the top rail is sitting below the barrel nut.
Make sure you snug up the screw, but don’t over torque it. Excessive force can lead to the rail cracking.
Finally pull back the slip ring or delta ring once more and side the lower rail section into place. Then you are done.