Think what you will of the price of Surefire's flashlights, they are stout. Double O-rings, heavy aluminum body. Shock resistant lens (it better be, being a rifle light and all).
So I got myself a Surefire M951 the way all Surefire lights should be purchased: used and cheap. It's been painted, kinda beat up, and likely sat unused and unloved for a long time.
The original Surefire P60 light module has become a defacto industry standard form factor for LED modules; but the original as came with the M951 is the old Xenon bulb style that ran on 6 volts and only put out about 65 Lumens. Absolutely archaic by today's standards.
I believe this (and most lights of this era) light was introduced before the 18650 was, or at least before it gained widespread adoption. So the tube is sized for two CR123 batteries, which will make a nominal 6 volts at around 1500 milliamp-hours. Compare this to a Panasonic 18650 and we've got 3400Mah to work with.
So this light gets two upgrades: first, replace the old bulb with a proper LED drop-in and two, bore out the light to fit an 18650 battery.
Until some parts arrive, here's the light all taken apart:
Many thanks to the folks at candlepowerforums.com for answering my questions and providing endless information on rabbit-hole of modern flashlight design, construction, and function. I've settled on a Malkoff M61 drop in, specially designed for Surefire upgrades. The specs say it runs 3.4-9volts input, and below that drops into direct drive. So that will be most of an 18650's capacity before it begins to taper off. I try not to let my 18650's go below 3.3v anyway, so they last longer.
You might notice this looks quite a bit different than most P60 modules on the market. I expected to just get a cheap drop-in, but reading about thermal and shock considerations in a weapon light lead me to this. A fully potted module in a solid brass body. Fitted to the flashlight head this provides excellent heatsinking for the LED and the potting completely encapsulates the electrical innards to prevent damage from recoil and vibration.
It uses a Cree XP-G2 putting out 450 Lumens. Quite a step up from 65.
Now that we've established that the Malkoff drop-in is awesome, lets give it some more power to work with. Now most folks might ream or drill out a flashlight body to 3/4", but in measuring this one I was concerned by the threaded area at the tail cap. The minor diameter of the threads is 0.840", so boring that out to 0.750" would leave only about 0.045" wall thickness. "Is this too thin?" I wondered.
Well after I realized I should go measure a couple other flashlights I no longer worry. My Olight M21 is a 0.74" internal diameter with about a 0.83" minor diameter on the threads, and it feels plenty stout. All the same I'll keep things good and concentric in the lathe here and bore it out to 0.74", that's about 10 thousandths clearance around the battery.
The rear portion of head that holds the P60 module in place also needs bored, as the battery sticks into it about 3/8".
I had figured to repaint this light a nice desert color instead of leaving the original, boring black anodizing. I had forgotten that some older Surefire lights had a nice gun-metal color of anodizing; I'll leave it at that.
Ordinarily I'd quickly strip the paint off with the bead-blaster, but I'm not sure if that would cut through the anodizing. So I pulled out the O-rings and donned the nitrile gloves and went to town with the paint stripper. I didn't touch the rubber tailcap though, I think anything I have that could strip the paint off would probably destroy the rubber switch.
So here it is, cleaned up and ready for action. With an extra 375 lumens and over double the original battery capacity.
Something I must stress first: these pictures really only work in relation to one another, and hardly look at all like what the lights do in person. I'm no photography expert, and it's very difficult to get a camera to 'see' exactly what a human eye sees, especially in the dark.
It's difficult to see the treeline in these photos, but it's about 60 yards away. To my eyes the malkoff module and Streamlight TLR-1 light reach about the same distance, and are about as bright at 60 yards at the very center of the beam. The big difference is that the Malkoff keeps that light very even through most of it's width, and it has a bit wider beam than the Streamlight as well, which exibits more of that center hot-spot + surrounding doughnut look.
The original 6 volt xenon bulb (65 lumens):
Now, the newly upgraded m951 with it's 450 lumen Malkoff drop-in:
Next, a Streamlight TLR1 claiming 300 lumens:
Another thing I noticed about the Malkoff is it's very consistant and even spill. It has a much smaller reflector than the stock P60 module design and I figured this would result in a narrow beam. Glad to be wrong! The smaller 7/8" reflector has a wider spread and less of a 'spot' effect than the TLR-1 reflector. This is a good lesson in how reflector design can have almost as much effect on the performance of a light than the LED itself.