Greetings My Good Hombre
The loading data you cited IS correct!
the simple answer to your question "why more powder, less pressure?" is ......
welcome to the world of SCIENCE!
yup, I hate to tell you but reloading is a combination of the advanced sciences we loved to hate back in school..
to wit:
- inorganic chemistry
- physics of
solids, fluids, gasses,
advanced exploration of combustion and expansion of gases
- mathematics
- metallurgy
In this case, it is not too complicated, it has to do with rate of combustion ( burning the powder to turn into expanding gasses) and rate of expansion
of the gasses INSIDE DIFFERENT VOLUMES!
.357 26.2 gr H2O (1.70 cm3)
.45 colt 41.60 gr H2O (2.696 cm3)
the case capacity of the .45 Colt is approx 1.6 times the volume of the .357
thus when the powder burns it has MUCH more room to expand into , even when
it hasn't yet moved the bullet.
Thus the pressure is initally lower in the .45 because in this "initially closed system"
the gasses have 1.6 times the volume to fill just to get started.
Once the bullet starts moving the gun is no longer a closed system, because the volume the gasses
have to fill includes first the cylinder, then the barrel.
Then at any point in time one chooses, the volume of the .45 is getting larger faster:
V = π r2 h
example: at 2 inchesfrom the breech ( or, technically the internal head of the cartridge)
the .357 volume = pi * .357 * .357 * 2 = .800 cubic inches
the .45 volume = pi * .452 * .452 * 2 = 1.283 cubic inches
at 4 inches
.357 == 1.600
.45 == 2.566
each time the .45 is still about 1.6 times the volume of the .357, so we can see that
the volume increases "linearly" . That makes pressure calculation a tad easier.
Unless the powder burn rate and powder gass expansion rate is nonlinear! If it
was non-linear ( ie exponential, logarithmic, etc) the pressure calculations get messy
and instead of simple algebra and basic calculus, I would probably be ion over my head.
Technically, we should also take into account the powder space - the space occupied by the powder decreases, as the powder burns, thus increasing the total volume.
But Wait! The good news is, we can rely on ENGINEERING and measure the pressure with tools
and come up with tables of EMPIRACLE REAL WORLD RESULTS!
Oh, those are called loading manuals!
I believe this stuff is covered ( and explained much better) in the "technical" sections of
several of the best Lee, Lyman, and Speer loading manuals .... the parts that everybody skip.
Also, as time goes on, powder companies change formuals or methods of measurement so, load data
can change... for example my 1975 copy of the Lyman Loading book has a number of slight changes
from my 2014 copy...
Thus, it is ALWAYS a good idea to buy and read good current load manuals!
hope this helps!
prf marvel