In a surprising reversal that follows years of effort to design a one-of-a-kind commando rifle, the U.S. military's Special Operations Command has abruptly decided to abandon the new SOCOM Combat Assault rifle – the "SCAR," as the rifle is commonly known – in favor of previously-fielded carbines.

Details provided exclusively to reveal that SOCOM, the Tampa-based command that oversees the training and equipping of SEALs, Green Berets, Air Force Special Tactics Teams and Marine SOC groups, will stop purchasing the 5.56 mm Mk-16 Special Operations Forces Combat Assault Rifle and might require all units who now have them to turn the new weapons back into the armory.

Read more about the Mk-16 SCAR cancellation in Kit Up!

"The Mk-16 does not provide enough of a performance advantage over the M-4 to justify spending USSOCOM's limited … funds when competing priorities are taken into consideration," officials at USSOCOM said in an email response to questions from "Currently, three of USSOCOM's four components receive the 5.56 mm M-4 from their parent service as a service common equipment item." (Naval Special Warfare Command is the only component that does not purchase its weapons with Navy funds.)

SOCOM said it will instead purchase additional Mk-17 variants that use the heavier 7.62 mm round, more Mk-13 Enhanced Grenade Launchers, and a newly-designated Mk-20 Sniper Support Rifle. (Industry observers say the Mk-20 is basically the Mk-17 with longer barrel and other sharpshooter enhancements.)

News of the cancellation of the Mk-16 variant of the SCAR is a major reversal for a command that spent six years and millions of dollars fielding a rifle specifically made for use by special operators. It was the first rifle since the M-16 that was competed, tested, and built from the ground up for the military.

This cancellation will certainly be poorly received by program advocates who touted the weapon's mission flexibility, better gas piston operating system, and performance in dusty environments as clear advantages over the current M-4.

Elaine Golladay, spokeswoman for FNH-USA, the weapon's manufacturer, declined to comment on the cancellation for this report.

Ironically, the company announced May 4 that it had passed the final hurdle from SOCOM's weapons buying office to go into full production and fielding of both the Mk-16 and Mk-17. It is unclear if SOCOM had made the decision to cancel their buy of the Mk-16 when FNH-USA issued that announcement.

Additionally, sources tell that SOCOM is leaning toward requiring that all Mk-16s currently fielded be returned as retaining limited numbers of them would complicate training and logistics support.

Officials with SOCOM said the services have so far fielded 850 Mk-16s and 750 Mk-17s throughout the SOF community, but did not specify which units got what rifle. As of last count, reported Army Rangers, most SEAL teams and Naval Special Warfare Combat-Craft Crewmen had received a mix of Mk-16s and 17s.

Original program documents from SOCOM show a requirement of over 120,000 Mk-16s and nearly 40,000 Mk-17s.

It is unclear how many Mk-17 rifles SOCOM will buy. The command budgeted $3 million in fiscal 2011 to purchase SCAR variants and had an additional "unfunded requirement" of $1.6 million for SCAR.

"The Mk-17 fills the existing capability gap for a 7.62 mm rifle," officials said. "USSOCOM is in the process of determining the exact quantities of the Mk-17, Mk-13 and Mk-20 variants that will be purchased."