FORT SILL, Okla. (Dec. 14, 2017) -- The Fort Sill Logistics Readiness Center (LRC) Bldg. 2283 repair facility was full of sounds of triggers clicking, bolts sliding, and barrels being swabbed Dec. 8.

Parts were swapped out; a tech operated a green glowing laser silently etching a receiver, and another technician used a gauge to check the specifications of a weapon that she just reassembled. With so many people, there was amazingly little talk, as technicians focused their attention solely on their craft.

A group of 24 small arms repair technicians is on temporary duty at Fort Sill, converting 3,600 M4 carbines to M4A1 models. One of the enhancements of the conversion gives the rifle a fully automatic firing mode.

The upgrade is the Army's effort to convert all, or pure fleet, its more than 500,000 M4 carbines to M4A1, said Bernard Arellano, M4 Product Improvement Program (PIP) mission lead, who is with the Detroit Arsenal. The M4 PIP, run by the Army Tank-automotive and Armaments Command (TACOM), has been going on for three years, and about 150,000 rifles have been converted so far.

Mark Malone, LRC quality assurance evaluator, and Fort Sill's liaison with TACOM, said the conversion is on schedule and the work should be completed by Dec. 20.

"It's going good," Malone said. "All the military units have been cooperative and on time, and there have not been any issues."

The majority of the rifles are from the post's Training and Doctrine Command, Forces Command, and Installation Management Command units About a couple hundred belong to the Oklahoma Army National Guard here, and one local Army Reserve unit, Malone said.

The conversion crew is from Anniston (Ala.) Army Depot (ANAD) and has been at work since Dec. 4. The craftsmen are small arms repairers, job series WG-6610, said Chris Cotten, ANAD small arms repair supervisor.

Malone began working with the TACOM and units here a couple months ago for the conversion. Soldiers from military units bring in their M4 carbines to the LRC at about 6 a.m.

"The rifles have to be clean and have the slings removed and can't have any optics on them; it's just the basic weapon," Malone said.

Serial numbers are verified with the rifles' accompanying paperwork. Since the weapons are not signed over to the LRC, Soldiers stay with the unit's weapons during the entire conversion.

The small arms repairers begin about 7:30 a.m., intent to convert 300 rifles in seven hours, Arellano said. So basically it is just a one-day turn around for a unit's M4 inventory, said Malone.

Drill Sergeant (Staff Sgt.) Derek Lorenz, and Spc. Milagros Covarrubias, supply specialist, both with B Battery, 1st Battalion, 40th Field Artillery, stayed with their unit's basic combat training weapons Dec. 8, at the LRC. They brought over 120 M4 carbines.

"I'm excited to see the full auto option," said Lorenz. "My primary job is infantryman, so this is my bread-and-butter."

When weapons arrive they first receive a complete technical inspection, Arellano said. A rifle is disassembled; laser etched to reflect it's now an M4A1; reassembled, gauged to specifications and inspected; go through trigger pulls; and then given a final inspection.

The technicians rotate through the various works stations, but Cotten said he always keeps some of his most proficient workers at the final inspection station.

"It goes through five sets of eyes during the conversion," Arellano said.

The conversion replaces the barrel, the complete trigger mechanism, and the buffer, Arellano said. "We add the ambidextrous selector safety (for left- and right-handed shooters)."

Since the M4A1 is fully automatic, it requires a thicker, heavier barrel than the M4, Arellano said. The M4 can fire semi-automatic, or a three-round burst.

The M4A1 barrel length is the same as the M4, but the more robust barrel adds a little weight to the weapon (less than 1 pound), Arellano said.

The conversion doesn't change the characteristics of the weapon. "It fires the same, it shoots the same bullet (5.56mm), but can do it fully auto," Arellano said.

He estimated that the cost to convert an M4 to an M4A1 is between $200 and $300 cheaper than purchasing a new M4A1.

"By doing this we're saving the government quite a bit of money," he said.