I don't know of a specific reason for locating the interior bottom of the case because case walls aren't all designed the same. When I emailed Starline to find out if their .357 and .44 Magnum brass was identical to their .38 Special and .44 Special brass except for length, they said no. They make the walls of the magnum cases thicker near the head. So, if you seated the same bullet the same distance from the web in one of their Special cases and in one of their Magnum cases, you'd have a little more powder space under the bullet in the Special, and that space is what affects peak pressure.
The best bet you have for matching pressures is to find the actual case capacity under the bullet. Weigh a fired case, then fill it with water, push a bullet in to your usual COL (squirting water all over in the process), then remove the bullet and wipe off the outside of the case and weigh it again. The difference in weight is case water capacity in grains for your load and it is really that number that you want constant for same-weight, same construction bullets bullets with the same primer and powder charge.
There's a methodological caveat to the above, which is that lower pressure loads (below 30,000 psi, roughly) will often have their peak pressure depend on resized case capacity more than expanded (as-fired) case capacity. In the instance of making case measurements for a lower pressure load, you can seat a bullet into a resized case, weigh it, then use a glue dispensing syringe to fill it with water through the flash hole, and weigh it again (keeping it upside down to avoid spillage) to find the case water capacity. Case water capacity is distinct from case water overflow capacity; the former being water capacity in grains under the seated bullet, while the latter is water capacity in grains when the case is filled level with the mouth of the empty case. QuickLOAD uses the latter and bullet shape and seating depth to calculate case water capacity, and that calculator is a third way you can get the measurement.
Originally Posted by wncchester
The nose configuration has no effect on powder charges. Construction nad seating depth/burn space will matter but not much unless you're loading on the ragged edge of a KABOOM. Starting loads are intended to be more than safe for any bullet or firearm type.
Nose configuration can affect seating depth and that affects powder charge. Sometimes a lot. I'm thinking, as an extreme example, of a 148 grain .38 wadcutter that has to be seated flush to the case mouth to feed in the Smith '52 or into a .357 self-loader or into some lever guns, verses a nearly same-weight 150 grain round nose bullet that sticks out and leaves a lot more powder space underneath.
Construction can also matter a lot. Reread SL1's second paragraph in post #11 above for a lab measured example.
While some manuals have very low starting loads that would do what you say, I think what you see most, as with Hodgdon's data, are starting loads that are only about 10% below maximum. That creates a 20% to 30% reduction in peak pressure, depending on the cartridge and components used. That's only just enough to compensate for the measuring errors that copper crushers can produce for identical components. You don't get the extremes often, but I've twice run into starting loads that were already at or near maximum in a particular gun, despite using matching components to those the manual author did.
Below is an example that's on page 119 of the SAAMI centerfire rifle standard. They sent cartridges from the same lot of M1 Carbine reference ammo to nine different facilities for copper crusher testing. They all agreed on velocity pretty well (about 3.5% difference), but got over 23% different peak pressure averages. So just about what a 10% charge would compensate for if the error were a 23% low reading of what was actually operating at the SAAMI MAP. In that instance the staring load would be at SAAMI MAP and the max load on the list would be near the proof load range, with some individual rounds occasionally exceeding the proof range if SAAMI's assumed 4% standard deviation is used.
The more modern conformal Piezo transducers that give you psi instead of CUP are about twice as tight, so if you have data you know is Piezo transducer tested, then you have a little more wiggle room, but not necessarily enough to cover a bad combination of component swap outs.
Anyway, the bottom line is that starting loads in data can be low enough only for starting with matching components. An example is the .243 Winchester with 85 grain bullet. In the Nosler Manual with their components the maximum load of RL19 is 43.5 grains. In the Speer manual for the same bullet weight the starting load of RL19 is 46 grains and the maximum is 48 grains. The Speer starting load is not adequate for a change to the Nosler component combination unless you don't mind stressing your gun unnecessarily.