There are drop in rail systems, sure! Almost none of them are free floated, beware of claims to otherwise, UTG is big about this problem. I'm less trusting of UTG rails now, to be honest. They are quite a bit heavier and bulkier then higher quality rail systems from other brands.
My UTG rail system ended up getting downgraded to airsoft rifle duty. Spending a little more on a Troy or MI would really net you some nice weight savings and bulk shaved off as well. You may pay double for it, but you will REALLY appreciate the difference.