Regardless of assignment, you must have a light. From searching that warehouse on an alarm drop to searching a crack house for a felony suspect, you must be able to safely do your job regardless of the lighting conditions. The Navy SEALs have a corollary to rule #1: "One equals none and two equals one." Simply stated, you must have redundancy in your safety equipment; this means at least two lights. It might sound like overkill, but Murphy's Law states that your equipment will take a dump at the most inopportune time.
Various manufacturers have developed flashlights capable of 120 lumens (the correct way to measure light output) or more in three-volt lithium battery configuration or rechargeable. LED (Light Emitting Diode) lights have extended bulb life into the thousands of hours. What kind of light should you choose? I like a small rechargeable flashlight for general purpose patrol duties. I then back up that light with a three-volt lithium light carried on the belt. This arrangement allows me to always have a light should the need arise.
Weapon-mounted lights have improved in quality while getting smaller in size. These lights-whether for a carbine, shotgun, or pistol-allow for increased accuracy potential while shooting. But in the case of a pistol, use your off hand to open doors, extend the arm for balance, or any manner of physical activity. Always remember that when using a weapon-mounted light: a) You are in fact pointing a firearm, not just a light, and b) You must have a backup light.
When weapon-mounted lasers first came out I was not a fan. The early designs were too big and cumbersome and the systems could be easily knocked off zero. The new mini laser designs, when used in conjunction with a white light, can be used in low light to increase accuracy, from behind cover, from unconventional shooting positions, by a wounded officer, etc.
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