Floating rails are very nice if you mount a lot of attachments to it, it will never affect the barrel. Remember on a real steel, the barrel is steel. On a airsoft M4, the outer barrel is alum and can be tweaked easily.
Each barrel nut supplied with the different rail requires a different size wrench. If you don't use a proper wrench, the barrel won't be properly tighten down.
A universal nut wrench that can be used on the buffer tube as well as any barrel nut in the future.