CQB, SBR, Mk18, and M4A1 Uppers — Short-Barrel Military Clone Configurations
The short-barrel military carbine upper is where CCC's expertise is most concentrated. From the Vietnam-era XM177 to the current-issue Mk18 CQBR and M4A1 SOPMOD Block 2, these upper receivers represent the close-quarters and special operations configurations that define modern American military small arms. Building them correctly requires knowing the exact specs — and knowing where to source the genuine components.
Mk18 CQBR
The Mk18 CQBR (Close Quarters Battle Receiver) is the 10.3" upper fielded by Naval Special Warfare Command, MARSOC, and numerous other special operations units. Built around the 10.3" Colt or Daniel Defense barrel with a Daniel Defense 9.5" rail, it became the default NSW CQB weapon in the mid-2000s and remains in active service today. CCC builds and sources Mk18 uppers with genuine Colt receivers, correct barrels, and rail systems that match the actual NSN specification.
M4A1 SOPMOD Block 1 and Block 2
The M4A1 SOPMOD program produced the most widely recognized American military carbine of the War on Terror era. Block 1 means KAC RAS rail on a 14.5" SOCOM barrel with FSB. Block 2 means Daniel Defense M4A1 RIS-II on a 14.5" SOCOM barrel, straight or FSP configuration. Both are correct, depending on the year and unit your build references. CCC has been speccing these since 2017 and the team knows every configuration point.
What You Should Know About SBR Configurations
Upper receivers with barrels under 16" are not themselves regulated — the lower receiver becomes the SBR when assembled with a short upper. A 10.3" Mk18 upper can be legally purchased without NFA paperwork; attaching it to a lower makes that assembly an SBR requiring Form 1 registration or Form 4 transfer. CCC can walk you through the legal path for any SBR configuration.
For clone-correct CQB and SOPMOD upper builds, there is no better source than CCC. We've been doing this since 2017 — contact us to discuss the right configuration for your build.