I have a cheap solution for you. I had the same problem with my Colt and fixed it using the following method--it works. Please excuse me if my use of terminology is a bit confusing:
If you remove your handguards, you will notice that under the delta ring, there is a sprocket-like thing with scalloped-out indentations. If you look on the underside of each handguard, you will notice that there is a small groove at the near end of the handguard with a raised bump or bumps in it. That bump is supposed to engage the scallops in the sprocket underneath the handguard and keep it from moving. Problem is, the bump is too small to stop the whole thing from wigglin'
What you want to do is take some alum. foil and fold it up so it fills the groove in the handguard. It will stay in the groove on its own accord once the handguards are put in place and the delta ring slipped over it. A little trial and error is required to get the right amount of foil in the groove--too much will prevent the delta ring from slipping over the handguard--too little will not stop the thing from moving.
If I remember correctly, the foil kept falling out of the groove when I put the handguards on. I stopped this by placing a piece of masking tape over the goove with the foil in it and trimming the excess.
My handguards are now nice and firm. Give it a shot and let me know if it worked for you.