Following dissatisfaction in the field with US subsonic ammunition, in 1970 contingencies from the U.S Army Ft. Benning AMTU/Sniper SChool and the USMC Quantico MTU/Sniper School approached Mitchell WerBell and Jim Leatherwood at Sionics/MAC to manufacture an XM-118 subsonic ammunition configuration they called "Drop Tube" ammunition.
The "Drop Tube" was composed of a Sionics/MAC XM-118 subsonic round inserted bullet end first into the throat of a fire M1 (.30-06) US made cartridge case with the resulting assembly inserted into a small aircraft aluminum tube capped with a waterproof NASA developed red composite sealer and marked on the end and head stamp with the with the tradition subsonic green paint.
It was the intention that the 7.62 drop tube round would be used with a Sionics M14SS-1 suppressor mounted XM-21 sniper rifle by Special Operations units that typically operated in remote locations far outside the range of Huey and Chinook helicopter evacuation.
Operationally, the sniper would fire on his target, retrieve his ejected XM-118 fired cartridge case and then drop to the ground the fired M1 cartridge case for the enemy to find and hopefully deduce came from an ARVN shooter.
The VC and NVA had become very proficient in identifying the head stamps of spent cartridge cases to determine the identity of the shooter and to plan their counter attack strategies accordingly. Major counter-attacks were seldom mounted against isolated ARVN sniper shootings, while suspected US sniper activities commonly triggered extensive search and destroy enemy activity.
The "Drop Tube" ammunition strategy was very successful in misdirecting many enemy search and destroy patrols enabling US sniper teams to escape and conduct sustained offensive activities.
The Leatherwood plant manufactured a total of 16,000 drop tube rounds that were packaged into 500 32 round kits. Each kit was composed of 4 rounds packed in a polyethylene plastic CHIEU- HOI labeled bag inserted into a nylon slip sleeve case. Two slip sleeve cases containing a total of 8 rounds were inserted inside a M1967 M-14/M-16 nylon combination pouch with both of the pouch side grenade pockets removed to distinguish it from the standard M1967 pouches. Four M1967 pouches were then secured inside a specially built nondescript green painted heavy galvanized steel lockbox to assure maximum security and protection in transit and to promote sniper team anonymity.
NOTE: The CHIEU-HOI bags were originally designed to protect 20 round M-16 magazines from Vietnam's tropical monsoon climate, but were used universally to waterproof many items.
CHIEU-HOI in the Vietnamese language means "FREE REWARD" and yielded Dong or $$ for those presenting these bags to US forces and was used as a propaganda device to recruit Montagrad tribal informants and build VC counter intelligence misinformation networks within the villages and hamlets. It is rare for the polyethylene plastic bags of this type to survive as they were made using 1960s crude plastic technology that was chemically unstable when exposed to UV light that quickly reduced the plastic to powder residue.
Exact figures unknown, but only a tiny number of these drop tubes have survived to date.