If that's yours, Justin, very cool! :thumbsup:
I bought one of these Tacstar extensions approx. mid 90's. It was my first flashlight "mod". Unfortunately, I discovered a couple years ago that the alkaline in it had leaked, and wouldn't come out no matter what I did. It was a sad thing to have to toss it after 9 months of trying to save it (including 7 or 8 months soaking in vinegar to dissolve the dried alky gunk). :sigh:
Anyway, they came with their own bulb that was rated for the extra voltage because the stock MM bulb would blacken quickly, then blow soon after that. You can simulate this by running one on a 14500 Li-ion.
To answer Elseis66's questions, the positive end of the 3AA MM LED tube is designed differently than the incan, and doesn't look like your idea would work. Also, as mentioned above, the Minimag bulb doesn't respond well to overdrive. It's definitely brighter, but won't last long. You would need to find a T-1.5 size bulb at the right voltage. And, it would have to run cool enough to not melt the plastic switch/bulb holder, and reflector. More about this below.
The way to go is to use a donor Minimag tube for the extension piece, then find someone with a 5/8 X 28 die to cut the threads. A die is needed for this because the tube wall is too thin for machining. Here are a couple corroborating pics from my adventure down this path a few years ago. For further detail, check out
post #12 of this thread.
Machining attempts... the one that didn't break was the wrong length, and probably wouldn't have survived screwing it onto the end of a MM anyway.
This is the die-cut extension. I treat the light this is on very gently, so I don't know how strong it is. Still hasn't broken though!
Now, back to your main problem here. You may not be able to find a useful bulb, and the ones you're likely to be interested in will melt the stock parts. This is why the Auroralite Minimag hotwire kit was created (the old thread is in the Marketplace... will link when the MP is back up), but they haven't been available for a long time, and rarely come up on Custom & Mod B/S/T. If you somehow manage to snag one, I'd suggest skipping all the hassle of trying to get an extension made, and run a Streamlight TL-3 bulb on two 14500 Li-ion. It'll be MUCH brighter than stock, and give a good 25 minutes runtime.
Member Litho123 used to sell a custom halogen filled Carley bulb (the H1499), but only has a few left, and won't be making more. Very bright, but with no availability, this is not the way to go. The Fivemega FM1794, now sold by Nite and Wonderlite is
extremely bright, but 14500 Li-ion's cannot handle this kind of current draw. Rumor has it that AW is coming out with IMR 14500's, and this bulb would push even those batts to the edge... in addition to giving a ludicrously short runtime.
Again, if you can find an Auroralite kit, go with the TL-3 solution. Happy hunting!