I was wondering what the difference is between a free float tube and standard handgaurds.  Can you install one on a standard rifle.  I was looking at the DPMS 4 rail free float tube with rails already built in to the handgaurd for my LEGP.  How do these things work and are they worth the extra cost?  

I installed a DCM legal Armalite free float tube on my Bushmaster XM15E2S (i.e., the standard model). "DCM legal" means it fits under the standard handguards, and doesn't change the outward apperance.

If you don't free float the barrel, your point of impact will change depending upon the force you apply to the sling or front handguard. This makes a big difference, specially if you shoot using a sling. I imagine it will also make a big difference if you use a bipod.