Is this the rail for rifle length ar 15s?
Yes.
https://www.gunbroker.com/item/979812132
The KAC M5 RAS (Rail Adapter System) was developed for use on the US military?s M16A4, and with hundreds of thousands of rails issued to date.This non-free float handguard is a simple-to-install, drop-in upgrade that features genuine Mil-Spec M1913 Picatinny Rails along its entire length. What sets the M5 RAS apart from other 2-piece quad rails on the market is that it securely locks to the rifle?s barrel nut via a clamp and screw arrangement, making it a very solid foundation upon which to mount tactical accessories.
Rifle Requirements: The M5 RAS is a drop-in replacement part for standard, 2-piece polymer, A2-type handguards. The rifle must have a rifle length gas system, delta ring assembly, fixed front sight/gas block, and either a round or triangular handguard end cap. Lighter barrel profiles work best (Government contours). Heavier barrel contours may require modification to the inside of the rail. For AR-15/SR-15 based platforms only; not compatible with SR-25 variants.