A Radioshack Halogen HPR52 is a 2.8V 850 mA bulb that generates 2.78 MSCP or 35 lumens when alkaline batteries are fresh.
When the alkaline batteries are somewhat used or if you use NiMh or NiCad rechargeables, the Radio Shack XPR102 Xenon bulb is a 2.4 V 850 mA bulb that generates 2.10 MSCP or 26 Lumen. ( More light will be generated above 2.4 V, but Xenon bulbs don't like to be very overdriven.)
A More power hungry Krypton 2.4 V 900 mAh bulb generates 1.30 MSCP or 16 Lumen. It is more tolerant of being overdriven by new batteries.
If you want to extend battery life by more than 50 % over the 900 mAh bulb while dropping brightness only 23%, use a KPR101
which is rated at 2.4 volts and 600 mA.
I believe that the stock Mag 2D bulb is a 750 ma Krypton bulb.
A Carley 2.5 V 1.3 A Krypton Star in my Mag 2D with Nicads. It generates 3.3 MSCP or 41 Lumen. This will give maximum brightness for the shortest time with batteries that need to be recharged periodically.
