Shimming Gas System.

t44fan: you say that your front guard has fore-and-aft and rotational movement. Are you talking about the handguard or the gas cylinder or the barrel band?

The fix for the handguard is about half an hour's work with acraglas gel or other favorite bedding compound, in order to get a tight but removeable fit up front and a tight upward push against the handguard retaining clip in back. The result is a handguard which can be moved, with difficulty, back against the receiver but when slid forward, will not move back under recoil. And it won't rattle.

The gas cylinder shimming can be done, as AFront says, without removing the flash supressor assembly or the gas cylinder. But each of those may have some looseness issues which you might want to address by peening the barrel splines where each of the parts set. For the gas cylinder, peen only the top spline to better seal the cylinder-to-barrel fit.

Shimming is generally done in order to get the gas cylinder lock to "clock" to a proper tightness as it is aligned with the gas cylinder and the gas plug. This lock is reversable and screwing it on from the other side may give a better lockup. But you have to remove the flash supressor in order to try this.

Shimming will also tighten up the fit of the barrel band to the gas cylinder if you choose not to unitize these parts. .015" works on many rifles, gives a good lockup for the gas cylinder lock without interfering with the barrel's gas port. .015" is two of the Fulton kit's shims IIRC. I'm curious about how thick an ACE hardware retaining clip might be.