It is due to the distance from the cap to the control parting line and for the length of the spring. The total stack in A2 is 23, and the distance between the parting lines is 10, so 23-10=13, which is what is left after the bottom pins, master pins, and buildup pins get the chamber up to the control parting line.

That’s what I would like to know as well.

This is for if you pick the core out of the lock/housing, I had to pick one out then I reverse engineered a core key so I can reuse the housings. Have a keying block, a sfic kit, key cutter, and the wall of key blanks

It is possible to remove the core from some locksets when the door is already unlocked by disassembling the lockset from the door. The only reason to have to decode the control key is when taking over a set of locks and the paperwork is not available from the previous locksmith. Decoding cores is not a primarily technique for entry, although picking an SFIC core to control is not a huge secret.

​@@kenchiltonI recently took over a set of locks and although most of the paperwork is available, not all of it is. So far I have found 3 different controls and am aware of a 4th. I've only had to drill out one lock so far, but will probably have to drill out a lot more before I'm done. Hoping to minimize that, so I'm considering learning to rake & pick.

Yes, sort of. If it is master keyed, you will be able to identify many keys that will open the lock this way, but you won’t know which are the masters and which is the change key without decoding more cores.

Another comment answered this already, before you even asked, but I'll reiterate it. In an A2 system all the pins in a chamber (the pin stack) add up to 23. The distance between the change key shear line and the control key shear line is 10. 23 minus 10 = the magic number 13. Why does 13 mean anything special? The pin stack always adds up to 23 (on an A2 system) and the distance between the two shear lines is fixed at 10 so only the bottom and top pins will vary to add up to the necessary 23. If we have bottom pins that match our control key then what remains is the top pin which equals 23 - 10 - bottom pin/control key = top pin i.e. 13 - control key = top pin or for our purposes 13 - top pin = control key. The caveat to all of this is the requirement that the lock was properly pinned up by the book to A2 specifications. If someone chose random top pins, the lock can still work but this technique of measuring only the top pin to determine the control key will not work. In such a rare and unlikely case, you would have to measure the entire pin stack and use the formula: (control key cut + 10) - plug total = build-up pin. Plug total being the sum of bottom pins and master pins in a chamber.

Then it is not an A2 system or the lock is not a BEST style. In an SFIC with A2 numbering, the length of the spring and the distance from the control parting line to the cap are fixed dimensions that make the longest top pin that reliably works a 13. Longer pins are only used for buildup.