Since this is being discussed in another topic, I thought I would re-post this as the topic comes up a couple times a year.

An interesting but rare Retro accessory that is seldom seen is the Low Light Level Sight System (LLLSS). These are the luminous front sights with the corresponding large aperture rear sight as seen in most of the available M16A1 Operator’s or –10 Manuals floating around. Though they are often sited in the TM’s and would appear to the casual observer to be a common accessory, they are rarely encountered in the flesh.

These sights were installed and removed at the Direct Support (DS) level of Maintenance when they became unserviceable or outdated and required controlled disposal due to the radioactive isotopes. As such, they are rarely encountered for sale. I have spent most of my career at the DS level and have worked on most variations of the AR system and I cannot recall ever encountering one on a fielded rifle, if that is any indication of how rare they might be. Though I do have one set in my collection.

I’ll provide a brief description to go along with the pictures: The front sight is composed of a tapered round post that is larger in diameter than the standard A1 post. It is even wider than the later A2 square post. This is necessary to allow for the hole that is drilled into it from top to bottom that houses the vial of illumination isotope. The sight is then slotted to expose two sides of the vial. Since there are only two sides exposed to the operator, the sight has four adjustment notches like the later A2 post. After making adjustments to BSZ the weapon it may be necessary to go an additional click down to expose the vial to the operator. The base of the sight post is marked with the Radiation symbol, MFR date, Isotope type and quantity with the markings divided by the adjustment notches.  

The rear sight roughly resembles an A2 aperture in that it has both large and small apertures. In this system the large aperture is unmarked, has a diameter of 7mm and points towards the muzzle when folded down. The rear aperture is the L marked aperture found on conventional A1 sights with a diameter of 2mm. It is used to apply the BSZ to the weapon and is the primary aperture out to 250 meters. Anything over that requires hold over as the large aperture is only used for low light, or extremely close range work.





I’ve researched various manuals dating from as early as 1966 to the last M16/A1 manual printed in 1991 in an effort to try and tie down a timeline when they first appeared and ascertain their appropriate use as it may relate to Retro.

DA PAM 750-30 dated 1 July 1969 makes no mention of these sights. This is the first of the small Operators Manual that resembles a comic book. These were printed to at least January of 1972 as I have several sealed in their original plastic wrappers having this date.

TM 9-1005-249-20 or the Organizational Maintenance Manual dated September 1971. This manual makes no mention of these sights either. These two pieces of information are kind of disappointing as they would indicate that these sights were not available during the ’65-’72 timeframe or, the bulk of active ground fighting during the Vietnam War.

The first documented mention of the LLLSS that I have found appears in FM 23-9, M16A1 Rifle and Marksmanship Manual dated June 1974. The luminous isotope is listed as Promethium and it makes reference to it being a limited issue item but makes no specific mention as to what type of units may receive them or the density. In addition to providing a description, zeroing instructions and doctrine for their use, the FM provides some warnings for the care and maintenance of the units that has some relevance as to why they are rarely encountered today. Of most interest is the warning not to blacken the sight using heat or to use a metal bristled brush to clean it as the vial containing the Promethium is made out of plastic and susceptible to damage.  

The LLLSS in my collection is a Promethium unit marked Pm 147, 1 mc, dated 03-73. This would indicate that they were manufactured and possibly issued before they were included in any official references at least as early as 1973. Another member (rlc AR15.com) has a Pm 147 set dated 08-74.

TM 9-1005-249-10 the small blue colored Operators Manual dated April 1977 and the one most commonly encountered containing the LLLSS, refers to the sights as Promethium and includes zeroing instructions. It also states the vial containing the isotope is made of glass, which would indicate a change in the manufacturing process to increase the durability of the unit.  

In July of 1978 the U.S. Army Armament Materiel Readiness Command issued a Request for Proposal for Tritium front sight post assemblies for the M16/A1 rifle.

TM 9-1005-249-23&P Organizational and Direct Support Maintenance Manual dated November 1983 states that Promethium is no longer available for issue and must be replaced and turned in for disposal utilizing procedures for radioactive materials. It goes on to mention the introduction of Tritium illuminated sites that are marked H3, 9mc, mfg date and having a glass vial containing the isotope. The earliest example of a H-3 site reported to this author is dated 09-80. It further states that Tritium sights have a service life of 144 months (12 years) and are to be replaced after exceeding this limit. More interesting is a paragraph stating that Tritium poses no significant contamination hazard (as opposed to Promethium) as the amount of Tritium contained in the sight (9mc) cannot be detected with issued radiation detectors.

TB 9-1000-247-34 Standards of Overseas Shipments dated July 1987 thru the changes of June 1991 state that the Tritium LLLSS sights are still acceptable for service.

The last mention of the LLLSS that I could find is in the Jun 1991 version of TM 9-1005-249-23&P, or the last published M16/M16A1 maintenance manual. In addition to the previously published information as to identification, replacement and disposal of the sights when damaged or exceeding their useful life is the statement that; The Low Light Level Sight (front and rear) are no longer being manufactured and are obsolete items. Though it gives no indication of when they were declared obsolete or last manufactured.

What does all this mean as far as the Retro builder is concerned?

Unless we can get some testimonials from veterans who may have used them, or documentation to support it, it would appear that they were introduced to late to see much if any use during the Vietnam War.

Due to the fact the front sights required special disposal procedures, they are rarely found with their isotope vials intact if at all. Since the rear sight apertures were not subject to the same requirements they are more likely to be encountered though are still uncommon. If anyone has additional documentation, first hand accounts or own examples in which to glean dates from to tie down periods of manufacture or transition please post them and I will add the information to the database.  

Summary:
Promethium (Pm 147) was used to provide the illumination from introduction in 1972(est.) to sometime before 1980 when they were removed from service. Promethium was more hazardous then Tritium as it was subject to stricter disposal and decontamination procedures. Promethium sights used a smaller amount of isotope (1mc) to achieve the same illumination as latter Tritium sights. Early versions of the Promethium sights were more prone to breakage/contamination than Tritium sights as the vial containing the isotope was made of plastic. A member (uxb AR15.com) reports, Half-life of Pm147 is 2.6 years and is a strong gamma-emitter with energy peak range at 121 KeV (kilo electron volts), which puts it close to Cobalt 57 with a peak of 122.  

Tritium (H3) replaced Promethium sometime between 1978 and 1980. The Army solicited for Tritium front sights in July of 1978. The earliest Tritium H-3 marked sight reported to the author is dated 09-80 (QUIET AR15.com). Tritium sights utilized 9mc of isotope (gaseous) contained in a glass vial to achieve illumination. The amount of Tritium contained in a sight poses no significant hazard of contamination if broken. Tritium sights have a service life of 144 months or 12 years. Tritium sights were still acceptable for service as late as 1991 though they had been declared obsolete. The date of last manufacture is unknown at this time.

Wpns Man

pics of my set.  my markings look like yours except for the date.  radiation symbol, 09-73, Pm 147, 1 M C

I have a 9/80 Tritium FSP new in bag that member QUIET was kind enough to give to me.

The DLA contract number in '79

http://www.thegunzone.com/556dw-4.html

Frankford Arsenal initiates the M16 Sight Enhancement Program to develop improved low-light sights

ETA:

After eight years of testing, Frankford Arsenal has evaluated sixty different sight systems from which five main concepts emerged. Two of these are considered significant improvements. The two concepts are early versions of iron night sights using "promethium" and non-magnifying "reflex sights." The reflex sight is considered superior. The leading model at this point is the Reflex Collimator Sight (RCS) designed jointly by Frankford's Fire Control Laboratory and AAI. The preferred reticule consists of three small yellow wedges configured in an inverted 'Y'.

http://www.thegunzone.com/556dw-7.html


July1978
ARRCOM issues a RFP for tritium front sight post assemblies for M16/M16A1 rifles.

Great write-up.

I've mentioned before that I don't agree with the board's opinion that "Retro" ends with the Fall of Saigon.

These LLLSS were installed on all of our M16A1s in the early 80's (US Army) and are a bonafide Retro accessory IMO.

Thanks for sharing info.

Been a long time since I read the timeline,

I must have overlooked that tidbit, thanks. It fits within published and observed data. Considering proposals from manufacturers, testing and contract approvals observed Tritium sights with mid to late 1980 dates would seem about right. Unless of course 1979 examples surface.

I have a blueprint and specs sheet at work for the front sight with revision dates. I’ll have to see if any of the revisions coincide with observed changes. But it will have to wait as I’m on leave until the end of the month.  

Wpns Man