Alum gets hot very fast. If you try to hold an alum rail, it can burn the hands. If your weapon doesn't get that hot do to low amount of firering, the alum notches will hurt the hand. If you cover the alum rail with a panel the alum notches won't hurt the hand, but then the heat going into the alum can't cool very good since you covered and trapped the heat. If you are going to shoot rapid fire, then the panels will protect the hands to a point and then the panels can finely get too hot, and a forward pistol grip is needed. If you go to a non alum hand guard with vents then you are less likely to have the above problems. If you figure out what type capabilities and shooting you want to do, then you can compare ability's of a rail system to what your buying one to do in the first place. Your on the right track with a Free Float.
Good shootin, Jack