Get a Gibbz Arms Forward Handguard, they feature a design that doesn't require any fooling around with a spline nut because they don't use a spline nut. What you do is screw the mounting adapter for the handguard to the upper with a bit of locktite blue, make sure that adapter is perfectly aligned with the upper, and finger tighten the internal nut and wait 24 hours till the locktite has set. Then you use a 1 3/16 crowfoot and torque wrench to torque that internal nut to 40 ft.lbs. After that you mount the handguard and you are done. Note, in the event you have to break loose that adapter to mount a new barrel once you have the internut unscrewed you will need to heat the mounting adapter to about 150 degrees (a large pot of boiling water to "steam" the junction will work well) and you should then be able to break the locktite loose.

Final note, the Gibbs Arms handguard range between about 120 and 190 bucks so they are not cheap but they do come with everything needed except the crowfoot or torque wrench. For the crowfoot go to pts-tools.com, they sell a 1 3/16 crowfoot for something around 22 dollars, actually they have a snap on for well over 100 bucks and an "import" for about 22 bucks. I have the "import" and IMO it's the exact same part as the Snap On except it isn't stamped Snap On, it's unmarked.

Finally I have found the Gibbz system so easy to use I now have 3 AR's with their handguards and a co-worker who got me into building AR's has now put the Gibbz handguards on two of his builds. They aren't cheap and are sometimes out of stock in some lengths but they are an EXCELLENT design.