You are correct that the most accurate oal measurement is from the ogive to the base of the case.
Most bullets, especially HP and SP bullets will vary as much as 0.020" in length and make it very difficult to get an accurate oal measurement if it is taken from nose to case head.
The tipped bullets are much more consistant but still I have seen variations of close to 0.010" from different nose lengths.
To be honest, I have yet to see this make any real difference in accuracy as long as the distance from the ogive to the lands is consistant which it generally is quite good with most bullets.
Your opinion about factory rifles having the same distance from the bolt face to the origins of the lands is not correct in my experience. I have teken alot of factory rifles apart using the actions for custom rifles
. Most of these actions are new out of the box Rem 700 ADL rifles and many in the same caliber.
The throat length can very widely from one rifle to another, sometimes by as much as 0.050" or more.
The problem lies in the fact that a company like Rem or any other large rifle maker will have dozens of reamers for a given caliber.
They also will generally get as many chambers reamed with a given reamer as they can before resharpening the reamer to save on overhead.
Well, a fresh new reamer will cut a different chamber then a reamer that is on it 20th chambering job.
Also, the folks that do the chambering are not what youwould call custom gunsmiths. They have a quota of rifles to put out in a day and this does not lead to real precise chambering or any other machining for that matter.
If you get a rifle that was made on a Tuesday morning, you are much more likely to get a better machined rifle then one that was finished just before the closing bell on Friday!
Headspace will vary from rifle to rifle as well. I have seen many factory rifles with leadspace measurements in the 0.010" range. This is within spec for a factory rifle but still quite loose in my opinion.
So if you take a quality chamber with headspace in the 0 to 0.001" range and compare that to a 0.010" headspaced rifle, your oal will vary the same amount or more or less depending on the reamer used.
Reamers are another variable, while they are close, they are not exact, one reamer to another, there are small variations which will also translate into chamber length variations.