Here's a pic of a 16340, 123A, and 18350 battery. The actual sizes are supposed to be 16x34mm, 17x34.5mm (about 2/3 of an A battery), and 18x35mm (half of an 18650+2mm). But the 123A Surefire is the same exact size as the 16340. To check if it was 1mm larger in diameter or not, I put it in my Perun Mini 2 and it fit perfectly. Even though it's only 3V and I shouldn't use it, I turned it on long enough to see that it worked. It's interesting to me that the 18350 although not being a large battery (they're each about 2/3 the size of my thumb) has 69-70% more mAh than the 16340.