I recently put a bulb from a Surefire G2 into a 4D cell Mag light. To do that, yank the reflector out of the maglight, stick a brass tack into the center spring of the Surefire bulb to contact the postitive terminal, stick it into the bulb holder and put the head back on, letting the bulb press against the glass to hold it in place.

If you use the P61 bulb, you get about 120 lumens. It's in a much larger package than the tiny G2, but you get very long run time on cheap D cells. It's no Surefire, but it really upgrades a Mag.

Nels