Re: a new rifle
I believe you're right, tolerances tightened up considerably, and Mobil 1 both! After 500 - 1000 miles, all my motors get is Mobil 1, and they still crank over in the winter too.
Lots of things in a motor that are fitted for a specific tolerance known to work, and there are "various" opinions as to what those are too. Same with an action.
I believe that every action needs to be measured in certain areas before work, some only after. Depending on how an action's threads are offset, off axis, mine may not be trued with the same thread as another.
If Chris were to follow a set blueprint, he'd probably be removing way more material at each step of truing than he ever needed to just to be sure that every action would true up with the spec's in those plans, and some smiths "might" just actually do this.
Most of it's like honing for pistons though, each one gets special attention as it may be slightly different, clearance is paramount.
Decking a block, line boring the crank and cam journals, correct warpage and core shift. After doing it though, you can produce excess pressure on timing gears or a slack chain, critical deck and valve clearances etc.
How one machining step affects other areas is accounted for, just less to account for in an action is all. Each one is really its own animal to be dealt with.