As far as I know, the use of "jam" came from short range benchrest, where shooters load at the range, and found it convenient to use it as a starting point for seating depth settings. The old school definition is the longest loaded length that a bullet can be seated to, using the neck tension that will be used for the loaded ammo, that will not be seated farther into the case as the round is chambered. The usual way that I find this length is to load a round (if I am at the range, and can can chamber the round at the firing line while the line is hot) or dummy round with the bullet seated long. The OAL is measured before and after chambering to make sure that the bullet was pushed back. I am usually working with a Wilson seater, and if I want to seat say .006 off, or shorter than jam, I will subtract the chambered length from the loaded length, add the amount that I want to be off jam to that and lengthen the combined length of the stem and cap by that amount, and record the measurement in my loading notes for the day, along with the rifle and barrel, and what bushing I sized with.
On the matter of load testing and seating depth, I usually pick a depth that has worked with that particular bullet, or one with a similar shape, and test powder charges until I come up with a likely powder charge, based on minimizing vertical, and then experiment with seating depth at that charge weight, in and our in increments of .002, all of this usually into the lands by some amount. I read about a lot of testing that seems to take too many rounds, and have seen threads where targets were posted that looked to me like the rifle had major issues like bedding, that were unresolved. IMO serious pursuit of accuracy requires that bedding be perfect before load testing is begun. I do not get the impression that this if fully understood by all that attempt it. There is also the matter of wind flags. Come to a short range benchrest match sometime. All of those flags are there for good reason. (Remember I am talking about load testing. I understand that short range benchrest flags may not be appropriate or available for other types of shooting.) People who choose to ignore this requirement tend to look to me like they have a lesson or two to learn...putting it as politely as I can. Elaborate flags are not required if one is on a budget. Sticks with lengths of surveyors' tape are a lot better than nothing. I also see some serious issues with bench equipment and technique, that are not a matter of intelligence or aptitude but rather simply an indication that the person has not been shown what works better. No one is born knowing this stuff. The reason that I mention these things is that I see fellows looking at their groups and thinking that they have a load issue, when it is probably the wind, their rifle's bedding, or something that they are doing at the bench while shooting. The good news is that these things are relatively easy to remedy.
