Here in our part of the word there is this device called caliper that is used to measure small cavity depth. Extend the stick till it hits the bottom, read numbers on the screen. But if you insist on pencil down the barrel, that's fine. 116 mm [116.02 take #1, 115.98 take #2] is my answer.
I would allow for a couple mm error though. New pencil, fresh eraser, the sucker was a bit bouncy against that breechface.
Please measure again. You gave the wrong answer. The correct answer is 120.1mm
Re measure and tell matt the right answer. Thank you.