"Bored" refers to the inner diameter of the battery tube. Stock, the C2 holds a pair of 123 batteries, also called 16340's in Lithium Ion chemistry. 16mm in diameter, 34mm in length. (Nominally the same size as a 123 primary cell) Boring the light out to 18+mm in diameter opens up the possibilities to use other cells. 18650's are 18mm diameter by 65mm long. (Nominally close enough in length to a pair of 123's to fit in the light) You will see all sorts of numbers related to lithium cells here. The search bar at the upper left will be your best friend. The amount of knowledge on this forum is phenomenal. Almost five years here and I don't have to look too hard to learn something new on a regular basis. The beauty of the Incandescent version is that it uses what are called P60 lamps, whose dimensions form the basis for a whole mind-boggling array of LED choices known as "P60 dropins" here. (Search that when you have some free time...) You can spend a career here trying to sort out all the available options for dropins from mild to wild. May be a steeper entry price on a Bored or stock incandescent C2, but the upgrade possibilities are endless. New tech in three years? No problem, someone here will more than likely make a new P60 dropin with that new driver/reflector/emitter you want...