The M500 weaponlight was part of Surefire’s “Millennium” product line, launched in the year 2000. (Although it is possible that they intended to begin production earlier, in Spring of 1998)

Before this, Surefire’s lighting solution for AR-15 platform rifles was their Tactical Light System, an ecosystem of modular mounts, adapters, and lights/lasers. This would later be called the “Classic” system by Surefire.

Shown: Model 460 Dual Tactical Light/ Laser Sight System for Colt Rifles and Carbines

The M500’s design itself was patented in January of 1999 (US6345464B1)

There are three overall versions of the Surefire M500 type light.

M500A & M500B	Carbine Versions of AR-15 & M16
M510A & M510B	Full Length AR-15 & M16 w/Round Handguards
M511A & M511B	Full Length AR-15 & M16 w/Triangular Handguards

The A and B sub-types represent the overall light output. Initially incandescent, the output was as follows.
A models could output 225 Lumens for 20min, or 125 Lumens for 60min.
B models could output 500 Lumens for 20min, or 250 Lumens for 60min.
These are reflective of the different lamp assemblies that could be interchanged. The A used three DL123A batteries, and the B used six.
Note the need of a special battery carrier.

While these stats may seem meager today, at the time they were impressive. The A series weaponlights provided over twice the light output of any previous Surefire six or nine volt tactical light.

According to their marketing at the time, “The A series provides the optimum amount of light power and the optimum beam spread for finding and evaluating suspects at distances typically encountered inside buildings or in built-up urban areas.”

“The “B” series weaponlights have ten times the blinding power of any previous Surefire six or nine volt tactical light…enough to completely, although temporarily, take away the vision of a suspect in low light, at conversational distances and beyond.”

The lights had momentary, constant on, and disable controls.

Due to the nature of the AR15/M16 handguards, the weaponlight could be installed upside-down if the user wished to keep the light from obstructing their view. This did mean the controls were in a slightly odd place however. See ‘Terminator 3’ scene below as an example.

Some models later sold had a special feature, a ‘Navigation Light’, which provided low levels of illumination, intended for reading maps and such.
These Nav Lights were available in Red, White, and Blue. These can be seen as early as 2002.

An M500L (“LED”) version was later released in 2009, increasing light output to 350 lumens which would operate for 1.5 hours. Interestingly, the LED enhancement first came to the MP5 and Shotgun dedicated fore-end lights before they released it for the AR15/M16 versions.

An M500V (“Vampire”) edition was also released in 2009, which allowed IR light to be emitted through the same lens.

In addition to making these weaponlights for the AR15/M16 platform, Surefire also released versions for the HK G36, G36K, (M560 and M570) and the Sig Sauer 551 (M520).

Initial MSRP for the 500 series was $480-$630 depending on model and variant. Later versions such as the M500V would be sold for $925.

The handguard light M500 series was delisted as of 2014 in order to prioritize their M900 series of vertical grip weaponlights, which were released in 2001, and increasingly used in place of the M500 series due to the proliferation of quad-rails. Shown below.

These days you can still find these lights on the second hand market for reasonable prices, but you may have a hard time finding the exact model you want.

More examples in real world and media: