Another thing about starting at jam that seldom gets discussed, but I feel matters, is starting pressure relative to neck tension. There's a point where it takes more pressure for the bullet to begin to move than what it takes to open the brass neck. Think about that for a minute...IOW, it takes most or all tuning related to neck tension out of the equation entirely. The biggest factor related to tension becomes, how far into the lands can I go with a given neck tension before I see it matter on target.Mr Murdica's procedure is pretty much the same as a lot of bench rest folks use, I think. My procedure is almost the same. You ask why not start .003" or so off the lands. First - his reference to using powder that fills the case is not to be interpreted as starting high on a ladder test. He is just trying to use a powder that most fills the case when load is developed as opposed to one which leaves a large powder void in the case - which many powder options will do - even at max safe operating velocities. The void will create erratic velocity. I'm, guessing he chooses to shoot "jammed" as opposed to off the lands for a few reasons. First in my mind is shooting jammed eliminates much bullet "damage" from the bullet otherwise hitting the lands slightly off-center due to concentricity issues or "slop". While some loads just don't like being jammed - it is not lost on most shooters that a lot of loads love it above all else- be it because it tightened up concentricity or because of other dynamics which made it the better load in that rifle. About 75% of my target barrels prefer a jammed load with the load I shoot in them. Maybe that is because I automatically go to jam as Mr. Murdica does and that enables me to see that the benefits of jamming are consistent enough to go there first - whereas someone who always starts well off the lands and never feels comfortable jamming just picks the best setting he came across. It is assumed by Mr. Murdica a REDUCED load is used initially, I'm sure. But a newcomer might interpret what you stated as just starting off jamming a bullet with a full case of powder. Not something any of us ever wants to do. You will encounter, as you say, velocity differences when seating bullets at different seating depths - but anytime a major seating depth change is made, an initial reduction in powder is warranted - especially when seating much deeper or going jammed..
That ought to way up the peanut gallery for a while.