The original M16 had a 20" barrel and a rifle length gas tube. The M4 has a 14 1/2" barrel and a shorter, carbine length gas tube. The length of the gas tube and barrel are designed to work together to keep the action cycling appropriately.
If you build an AR with a 20" barrel, you need a rifle length gas tube. If you build an AR with a 14 1/2" barrel, then you need a carbine length gas tube.
The issue comes up when you build an AR with a 16" barrel (a "mid-length" barrel). Where is the gas hole in the barrel? If that barrel is setup for a carbine length gas tube, then it will run, but recoil may be snappier. As a result, most people building 16" ARs will use a "mid-length" gas tube (and a barrel with the gas hole in the location appropriate for that tube). A mid-length gas tube is, not surprisingly, longer than a carbine length tube and shorter than a rifle length tube. Using a mid-length gas tube should make a 16" barrel AR run smoother.
Standard handguards connect to the upper at the rear, and the barrel at the front. A sling swivel attached to the handguards (or directly to the barrel) can cause the bullet impact to shift, based on how much tension is put onto the sling.
Free-float handguards don't touch the barrel. They are supported solely by their connection to the upper. So a sling swivel attached to the handguard won't change the point of impact even if sling tension changes. Also, if the handguard expands and contracts with temperature changes, that won't affect barrel impact either. As a result, free-float handguards improve accuracy.
Free-float handguards started out in NRA/CMP Service Rifle competitions, where you use a tight sling on the seated and prone strings, and need consistent accuracy regardless of sling tension.
