Are you talking about the gas cylinder being loose? The front sight attaches to the top of the gas cylinder, and the gas cylinder slides over the end of the barrel where it mates with three "splines" that surround the barrel and finally it's locked in place by the gas cylinder lock and screw.
My gas cylinder had some play as well, so what I did (and I can't remember where I read to do this) was put some light dents in the splines with a ball peen hammer. This solved it for me and as long as you're not removing the gas cylinder all the time you should be fine.
If it's not the gas cylinder then someone else might have some insights.