Good information. I am not sure of my chrono's window, but I believe it is a good bit larger than 3" x 3". Only testing will confirm that.
I envision placing the second chrono far enough down range to be able to discern significant velocity changes, but not necessarily at 120' down range. I did know that PBs do not track all that well, that was one of the highlights of this discussion. I do NOT know to what extent they wander, nor what to expect for group dispersion.
As for getting two data points per shot, that may not be a reasonable expectation. I do think that getting two data points for SOME shots (at some logical mid range for the second chrono) is somewhat reasonable to expect. The cases where two data points are recorded, even if the exception, should still provide very useful check points, something like inspecting random samples of a manufacturing process looking for trends in deviation.
For this experiment, those golden PBs that yield two data points will help confirm initial FPS base line numbers during the test series, as well as a definitive delta V, one of the prime questions the experiment is designed to answer.
Tower: "TWA 2341, for noise abatement turn right 45 Degrees."
TWA 2341: "Center, we are at 35,000 feet. How much noise can we make up here?"
Tower: "Sir, have you ever heard the noise a 747 makes when it hits a 727?"