6920/6940

To install a longer free floating rail you would need tools like, upper receiver action block, barrel nut wrench, and a shop table with vise.

The fixed front sight have to be shaved (trimmed) to low profile removing the upper towers, bayonet lug and sling swivel mount at the bottom of the sight, an alternative way would be just buying a low profile, but the long type to cover the holes of the old front sight base, it is recommended to have the new gas block pinned not just attached by set screws. Set screws even with high temperature threadlockers would loosen because of the barrel heat and shock.

If you're not willing to do it yourself you could send it to a professional gunsmith with good AR experience to do the installation for you.

Also to avoid all the rework take a look at some Colt AR already with factory installed rails that you might like:

Model LE6920 SOCOM II (with medium-heavy profile barrel limited production of 1500 carbines)
LT6720-R (with lightweight barrel) and 10" Troy rails
LE6940AE-3G longer rails than the 6940 and ambidextrous lower receiver controls (new model to be released at the end of this year - download the new 2014 Colt catalog from their website)

There are plenty of models with long modular handguards from Colt Competition Rifle (see their link below).

I installed a medium length 9" LaRue Tactical free floating rail on one of my carbines and several other rail brands for my friends and relatives, I've got all the required tools. Also I used my detachable carry handle sight to align the rail of the handguard with the rail of the upper receiver. It's not very hard to do as long as you have some average mechanical skills.


Colt Competition Rifle