- You wouldn't shorten a 6D Mag, you would just buy a lesser-cell Mag or body tube; cutdowns require sending the light out to a machinist and paying him for his time, a new light/host is cheaper.
- Alkalines give poor regulation but max runtime; You get 100% output when the battery is new, which falls quickly to ~25% in ~2 hours, but then the output will hang there for many hours, very slowly dimming to zero. This is why your Mag always seems brightest when you first install new batteries - it is. NiMH/Eneloop batteries give max regulation but mediocre runtime; output is 100% nonstop for a few hours, then instant death. Alkalines can eventually swell/leak, Eneloops do not. Alkalines will begin to get iffy at freezing temp, Eneloops will continue working to 0F.