Just so some of the other newer readers that may not be as well versed have some basis for the discussion going on in regards to this thread:
The barrel vibrates when fired. Mapped on
one of its axis it looks like this. This is typically what we are going to envision with a ladder test. A simple sine wave. This is what the ladder test is really trying to display.
That said a barrel vibrates in many ways. Each with their own sine wave pattern, so suffice it to say that and modeling we are trying to accomplish through these tests is really just a best guess.
I cant get the gifs to embed here but this link visualizes all the different ways the barrel can vibrate, worth a click to see the animations
https://www.varmintal.com/amode.htm
As you can tell from those animations trying to model this would be extremely intricate (a pain in the ass) but is what Optimal Barrel Time is basically trying to accomplish
http://www.the-long-family.com/OBT_paper.htm
This is probably more indicative of what the vibration sine waves would look like when all graphed together lol
So that brings us to the Ladders fundamental principle: Positive Compensation- basically we are looking at the initial single axis sine wave graphic I posted first and ignoring the others for general discussion. The fast bullets are coming out early in the cycle so they exit the barrel at a lower point of its vibration and the slower bullets take longer so they are in the barrel long enough to catch the highest point of the vibration.
So with a ladder test we are only really concerned about the vertical displacement at distance, in this example the 29.8 is slower and is released at a higher spot in the barrels vibration, the 30.4 is fast and is released at a lower spot in its vibration.
Ladders ignore left-right windage and rely on distance to minimize the shooters error and for velocity to be the main driver at comparing how the bullets shoot.
What about the other axis of vibration though? Thats where an OCW test comes in. It looks at not only vertical but also horizontal, and since we are focusing on the actual complete impacts 100 yards is the standard distance as its not really far enough for environmentals to play a big part yet still far enough to judge the rifles accuracy, everything will look good at 50 yards. It looks at not only vertical but horizontal as well. You look for where the groups impact compared to one another and try to find a stable spot where neighboring charge weights have similar points of impact.
If we were to take each of those individual targets, lay them over top of one another, and compare we would want to find a series of targets that have their impact centers close to their neighboring group so that it would look something like this hopefully.
