Welcome to CPF, Portland Jon!
I suggest you look at the Wolf Eyes Raider 9AX or 9HX. These have the same body, but the HX might be useful for you as it has a cable pressure switch which you can attach to your rifle's foregrip. You can then switch the light on and off just by squeezing the pad, without moving your hand. The AX version has a standard "clickie" tailcap switch (you can order a separate clickie tailcap if you want both options). Do NOT get the 9DX turbo head model as there are no Lumens Factory LAs which will fit in it.
You can order Wolf Eyes gear from PTS in the USA here:
http://www.pts-flashlights.com
Mike at PTS is very helpful. USA prices are much better and CPF members get a discount, but you will have to pay for shipping and (probably) UK Customs charges too. Still well worth it IMO.
Alternatively, as you are UK based, you could order from Wolf Armouries in London:
http://www.wolfarmouries.co.uk
However their website is completely useless and it is better to order from them by telephone.
Battery sizes: A good reason in favour of the WE 9AX is that it will take 2 x 18500 rechargeable Li-Ions or 3 x CR123A primary cells without needing to change the LA. Battery designations can be confusing, but the "18500" means it is 18mm in diameter and 50mm long (the last "0" means it is cylindrical). CR123A cells are 16mm in dia and 34mm long, so you can see 3 x 123s are the same length as 2 x 18500s. Sizes vary slightly from one manufacturer to another.
Battery capacities: 18500 cells will fit in WE bodies, but they will not fit in Surefires. If you particularly want a Surefire, you will have to settle for the slimmer 17500s or have the body bored out, which will weaken it. 18500 Li-Ions are ~1500mAh, and 17500s are ~1100mAh, so there is quite a difference in capacity.
Battery types: Lithium primaries are 3 volt cells. Lithium-Ions are rechargeables and are 3.7 volts. These are nominal voltages (as are most bulb voltages). Under a load like a EO-9 LA, which will pull a hefty 2 amps, CR123A primaries will sag from 3v to about 2.5v, giving ~7.5v for 3 of them. Fully-charged 18500s are in fact 4.2v each, not 3.7v. These will sag too, giving an average voltage of ~3.7v over their discharge period until they need recharging. Lo and behold, 2 x 3.7v = 7.4v, so the voltage from 2 beefy 18500 Li-Ion rechargeables is the same, on average, as from 3 smaller CR123A primaries. And the "9v" bulb is designed to take a 9v surge, but to operate most of the time on about…. yes, 7.5v! Clever, huh?
Protection: Make sure you get protected cells. Li-Ion chemistry is potentially dangerous (Lithium primaries have to be treated with care too, but that's another story). If you charge a Li-Ion above 4.2v, unpleasant chemical reactions start to occur in the cell. Above about 4.4v these reactions start to get really nasty and the cell can start venting violently, with flames and toxic materials coming out of it. At the other end of the scale, if you let the cell discharge to below 3v, it will be damaged. Below 2.5v it will usually be irrecoverable. If you have a good cell and a bad one in series, the good one will try to pump power into the bad one and this can result in a overheating incident. The solution to all these potential horrors is the protected cell. It has inbuilt circuitry which prevents overcharging, over-discharging, and large current drains. The circuit stops the cell going above 4.2v or below 3.0v, or from having more than ~4 amps pulled from it.
WE make their own protected 18500s (which they confusingly call 150a size). AW's cells are also excellent, and he has a sales thread on CPF here:
http://www.candlepowerforums.com/vb/showthread.php?t=140362
You will be able to read about other popular battery sizes and their specifications in that thread.
Good luck and good hunting!
