Meprolight M21 Reflex Sight Review

M21 Meprolight Review

a review by Mike Kupari

Disclosures: For the past three years, I have been using and evaluating a Meprolight M21 reflex sight.  I purchased it in 2017, on sale from Brownells, and have no affiliation or relationship with Meprolight (nor any financial relationship with Black Sheep Warrior).

The M21 is an Israeli-made open reflector (“reflex”) sight that utilizes dual illumination.  It has fiber optics to gather ambient light and a small tritium lamp to illuminate the sight in the absence of it.  It operates on the same principles as the Trijicon dual-illuminated optics, like the now-discontinued Reflex II sight, the ACOG, and the Trijicon Accupoint.

Optics like these were pioneered by Trijicon in the 1990s and offered several big (and relatively new) advantages at the time. This was obvious even before most users of such optics had even heard of the Bindon Aiming Concept or had heard the term “occluded shooting”.  Before LED lights were bright enough or reliable enough for weapons sights, fiber optics and tritium were providing both day and night illumination for them.  Even today, when some illuminated optics aren’t really what you’d call “daylight bright”, fiber optics have no trouble in this area.  In fact, some of them can be too bright; it wasn’t uncommon for soldiers in Iraq to partially cover the fiber optics of their ACOG scopes with 100MPH tape to reduce the intensity of the illuminated reticle.

Such optics also do not rely on batteries.  This is less of an advantage today than it was 20 years ago; today, red dot sights with 50,000+ hours of useful battery life are common.  Still, the battery life in most LED-illuminated rifle scopes can be pretty limited, especially when compared to fiber optics, which will work for as long as the sight is intact.  Tritium/Fiber Optic sights offer a level of simplicity and ruggedness in not needing batteries and not having any electronics.

Tritium, also known as Hydrogen-3, is the only radioactive isotope of Hydrogen.  It has a half-life of 12.32 years.  As Tritium decays, it emits beta particles (high-speed electrons ejected from an atom), a type of ionizing radiation.  While beta radiation can be harmful, the particles are generally unable to pass through most barriers and will travel only about six millimeters in the air.  Human skin will stop beta particles, as will the small borosilicate glass vials that tritium lamps are contained in.  Tritium is generally only hazardous if it somehow gets into the human body through an open wound or ingestion.

Tritium gas itself does not produce visible light.  When the beta particles interact with certain phosphor materials they will glow, but only dimly.  Tritium/phosphor illumination by itself is not generally useful during the day or amidst typical indoor lighting.

The downside to Tritium illumination is that over a period of years, the tritium illumination will begin to dim as a result of radioactive decay.  As Tritium decays, it turns into non-radioactive Helium-3, which does not emit beta particles and cannot be used to create illumination.  When the half-life of the Tritium in a sight has passed, the illumination will be roughly half as bright, getting progressively dimmer until burning out completely.  It’s worth noting that the fiber optics of a dual-illuminated sight will still function and that the tritium lamps can usually be replaced by the manufacturer.

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