Honestly, why do people use ultra high-current, high-output lamps? 20 minutes runtime on an M4 is not practical in most situations. The heat alone will make your hand sweat and now the flashlight is uncomfortable and slippery in your hand. What do you tell the bad guy when your batteries run down, "No, don't run/attack me, I have to change my batteries!" I'm sure there are valid reasons for using these high-output lamps, otherwise SureFire and the likes wouldn't make them, but unless if your SWAT or something, I just can't see it.