Oh, I didn't notice the r in the circle. ;) Ok, then a RCR123 is a 16340.
What size o-rings?
I tried a bunch of different sizes and ending up using 12mm x 2mm.
Mine look like separate millings, head and tail have a different finish and the milling on the tail is tipped as if the blank was not clamped correctly and driver contacts very worn( more than my month old stock mini I edc) Makes me suspicious we got our lights from the reject bin. Suggest you find a different manufacturer for next custom job Rey. It seems to be a different manufacturing culture there from production to advertising.
A CR123a is a lithium primary battery, ie non-rechargeable and has a voltage of 3.0v
A 16340 is also known as an RCR123, so they are the same thing. These a li-ion rechargeable batteries and have a voltage of 4.2v (there are many different chemistries available though).
16340 refers to the size, most li-ions use this measurement system. 16 = diameter (16mm) and 34 denotes the length (34mm) the 0 denotes the shape, as in round. These are only guides as some cells will vary slightly in dimensions. But it follows 14500 is AA sized, 14mm in diameter and 50mm long, an 18650 is 18mm by 65mm.
Back to the RCR123/16340’s, you can get a different battery type know as LifePO4, these are 3.0v and are a good rechargeable alternative to a regular CR123a for lights that can’t handle the higher voltage of the Li-ion offerings.
I WOULD also use them if Shaotech had delivered them 
Over 80 days gone, gladly I got my money back but they haven’t still arrived
I use Kumabear's unprotected 16340 DLG's - they seem to work great in mine, and at only $2.78 -- I know shipping overseas is a big problem... I also used the Yezl's but they are hard to find and expensive (eBay vendor in the UK had them) - I had 2 but gave them away with another BLF mini (my good mini, kept the bad one that has messed up threads from day one, flaky to get working right).
Are these still for sell?
30 left
I have tested the driver a bit and for me it appears to have some major flaws in software.
The low flash is well common.
And sometimes it has a 4th mode with no light output. The only way to remove this is to switch it on for some seconds, than turn it off for some seconds. Than it is the 3 mode again.
First I thought it is because of no perfect switch in the mini, but I have put my spare driver in a sk68 and now with a clicky I have these issue all the time.
With 2 different/same brand 16340’s I’ll sometimes get that “4th mode” (no light) too, but basically it’s only a 2 mode with low and high… with different brand CR123a’s all three modes.
I think you get the failure when turning on and then off than waiting some seconds.
It’s definitely a bug.
My BLF Mini seems to be getting worse - threads are bad from the beginning, they slip, and changing modes is difficult because you have to tighten it so hard so it doesn't flicker. Even so, modes skip - it gets very frustrating... Anyone having similar experiences? I'm using KumaBear DLG's unprotected exclusively - may have a NiteCore 16340 but I recall they were too wide and stuck. Think it's worth trying other battery brands? I'm ready to deposit this into the circular file, or donate parts to someone...
Try tightening the driver / pill with tweezers, cleanthe positive contact and contact surface from body tube.
Then report, if it helped 
I oiled up the threads twisted some times then cleaned up the threads and oiled again. After that procedure it twists like butter.
Maybe the mode switching is better, tightened a little, did clean the + contact. But you have to tighten it up so much before it engages... Gonna try adding solder to top of - ground ring on back of driver so the tail makes contact sooner when screwing in.... Let u know.
Update (2 mins later): Wow! That made all the difference!!! Added small bumps of solder, 2 on each side where the driver board kind of bumps out toward the edge. Now it's like butta, even betta than butta 
!! Really don't know if it's a permanent fix because the solder may wear down in time -- sometimes low mode is good, but not for me - I'm usually goin to high, and without memory, this takes a beat'n.
I know cleaning up the + contact helped because that seemed to remove the flickering. Also removed old grease and re-greased w/Nyogel.
It looks very consistent and now I got room to spare on tightening. Think over time, it will wear down, and tighten down further and further. But anywho, Wow!! Got a workable, great EDC back!! Also, a while back put in a XM-L2 T6 5000K neutral, and does about 465-415 (start-30 secs) lumens, just measured.
Where can a new guy get on board?
never mind I figured it out.
Mine was in the mail box yesterday . Thanks Rey, I like them very much. I think from order to mail box was around 2 weeks!
Great.