The "clue" is proving your theory wrong. Same weight and same seating depth have nothing to do with the volume of a hollow base bullet cavity added to the volume of a case, no matter how deep or shallow it's seated. Know anything about internal combustion engines? If you had a piston that only had valve cut outs and one that had a deep dished top, which would produce a higher compression pressure? Same weight, same travel, but smaller volume...
This is a silly argument, I'm done...
One last try. Real Berry's bullets. Plain base 148 grain wadcutter, and hollow base 148 grain wadcutter.
Same weight.
The two bullets on the left are Berry's 148 grain 38 caliber double end wadcutters. (The top and bottom of Berry's double end wadcutter bullets look the same.) The two bullets on the right are Berry's 148 grain 38 caliber hollow base wadcutters. Note the hollow base wadcutter on the right is LONGER than the bullet on the left, even though they are
the same weight and caliber (.357).
The 148 grain wadcutter is .555" long. The 148 grain hollow base wadcutter is .610" long.
How can bullets of the same weight be different lengths? Because lead has been 'removed' from the bottom center of the hollow base bullet (to form the hollow base) and 'added' to the skirt of the bullet's side (hence the term displaced).
They are seated to the
same overall length (see post 18),
NOT the same seating depth. Same overall length means the same overall length of the loaded cartridge, also called cartridge overall length.
Since they are seated to the
same overall length, the hollow base wadcutter will appear to be seated
deeper - if you're going by looking at the base of the bullet because the hollow base bullet is longer. This would suggest that the hollow base bullet takes up more space in the case, not less space. But it doesn't. The two bullet designs take up EQUAL SPACE in the case.
The difference in bullet length of the two bullets is because the lead was DISPLACED - moved from the central base to the bullet's sides (skirt of hollow base) of the hollow base bullet. The volume of lead is the same for both bullet shapes (it's just shaped differently), so they occupy the same volume of space in the case.