Colt is now selling upper receiver kits in various lengths. From the press release:

WEST HARTFORD, Conn. (May 8, 2014) – Colt’s Manufacturing Company, one
of the world’s leading designers, developers and manufacturers of
firearms, is now offering complete upper receiver assembly kits in a
variety of barrel lengths and configurations. The conversion kits are
derived from the popular LE6920 and LE6940 models, offering the
standard upper receiver with handguard configuration and the
monolithic one-piece receiver with free-floating barrel configuration.
Three upper assembly models come in the traditional 16.1-inch barrel
length, and for the first time, two upper assemblies are available in
the shorter 14.5-inch barrel length.

“The upper assembly conversion kits will allow us to offer more
high-quality, state-of-the-art choices to our customers,” said Joyce
Rubino, Vice President of Marketing for Colt’s Manufacturing. “The
upper assemblies are made to the same military specifications and are
subject to the same rigorous testing processes as every rifle that
comes off our line.”

All kits are ready to mount to a complete Colt lower receiver and
include a charging handle and a complete bolt carrier group with a
properly head-spaced bolt. Other features of the upper assemblies
include chrome-lined bolt carriers and barrels, with 1/7 right hand
twist barrel rifling. Each of the upper assemblies also comes with a
back-up rear sight. One model, the LE6920SCMCK, features a Troy rail.

An added benefit of the upper receiver assermbly kits is that they can
be used with any and all Colt lower receivers. Kits will all ship with
an adapter screw so that the uppers can attach to Colt models with
large pivot pins (.315” diameter).

The upper assembly kits will be available in Colt’s web store and at
local retailers. The MSRP will range from $740 – $1,056.

I doubt Colt will have any trouble selling them.