With most "free float rails", it's attached onto a barrel nut which in hand is attached to the upper receiver. Each rail system will have it's own barrel nut to mount the rail/hand guard. Some barrel nuts will require special wrenches to tighten them, but you'll have to be creative if you don't want to buy them. Most rail system are pretty solid and doesn't squeak like stock plastic hand guards. Depending on the length of the rail system you get, it shouldn't affect your iron sights. If you got a M4A1, you'll need to use a 7" rail to keep using your stock iron sight post. Do some research on the rail system you like and be sure to buy one that will fit and accommodate your sights.
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