I just got into using 18mm host flashlight bodies cuz they are cheap and I use 18650 rechargeables for my ecig, and thought I'd share this.
Just fyi, 123a's will fit in these hosts, but because they are bored to 18mm (to use 18650 batts) there is a little rattle if you shake the light. It is generally recommended to use primary cells in weapon lights, as they store better than rechargeables.
I wanted to mount lights on some long guns but didn't want to spend a bunch of $$. I got some solar force host flashlight bodies from lighthound.com and some led modules from customlites.com who is also known as "nailbender" on forums (he makes thermal epoxy potted led drop in's that are highly regarded but cheaper than malkoff's)http://www.lighthound.com/search.asp?keyword=solar+force&search=GOhttp://www.customlites.com/P60-Style-Replacement-Modules_c8.htm
You can add tape switches and all if you want, but I wanted to KISS
I wanted to use primary cells for my weapon mounted lights, but didn't like that they would rattle under recoil.
I googled around and found people using everything from rubber bands to stuffing 18mm tubes with paper.
I'm too anal to stuff it with some random material so I found oveready's tubes, which are 14 bucks each!
I tried orings, but none of the sizes at Home Depot would fit, they were too thick.
I kept looking around at HD and right across the o-rings I found this for $3 bucks and some change........
I wrapped it around two surefire and duracell 123's and slid them in my solar force l2m's. PERFECT!
The fit is just snug enough that they won't fall out on their own weight, but will if you tap or swing the light. Absolutely no rattle.
I also have the delrin sleeve that came with my solarforce P1 and like the spiral wrap fit better.
This set up is brighter than my g2, polytac, or tlr-1. the xpg g-2 has a nice combo of flood/throw and lumens/battery life.
High power led weapon light cost:
Solar force L2 host $12.99
Gun mount $4.99 (Note that this clamps on rails and does not have a bolt that goes through rail slots. I fired it on my AK for about 50rds and it did not slide down the rail. We'll see how it holds up.....)
XPG G-2 single mode $35
spiral wrap $3 bucks and change.
Shipping from both lighthound and customlites is cheap also.
Just for kicks.......here is another light I pieced together using two 18650's. Solarforce M3 head (3 mode), L2N host, and one 18650 extension tube. This thing is a beast with killer battery life!