You can run rechargeable batteries in a M6 but it is not Surefire endorsed.
The rechargeable battery configuration you pick also depends on what type of performance you want. Below are just a few of the more popular combinations.
An aftermarket 2x18650 battery holder for the M6 is the most versatile. It will allow you to run the 9 v bulbs such as the Surefire MN15 bulb for 2.5 hours at around 175 lumens which will give you a beam that is tight and throws well. The 2x18650 battery holder will also let you run the brighter bulbs such as the Surefire MN20 bulb and more suited to rechargeable batteries the Lumen Factory HO-M3T & EO-M3T bulbs.
For more output, the aftermarket 3x17670 battery holder will let you run the Lumen Factory HO-M6R bulb which is brighter than the Surefire MN20 but not quite as bright as the Surefire MN21.
If you prefer a LED solution, you can't beat the Lumens Factory Seraph P7 M Series Turbo Head which will just replace your existing M6 reflector. It will work with either the stock 123 battery carrier, the 2x18650 or the 3x17670 battery holders just fine.
Don't be afraid to go rechargeable. For the cost of around $75 (seven M6 battery changes), you can get two 18650 batteries with charger and a 2x18650 battery holder which will give you hundreds of charges.
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