Chris:
IIRC, the way Ruger bores the .45 Colt, and the dimensions are the issue.
For a long time .45 Colt guns were made with generous chambers to accommodate old ammo, and, since it operates at such low pressure, this is not an issue with modern brass. It is possible to run the .45/44 Colt Ruger at near 454 specs, provided the chambers are tight, and give little room for the case to expand. The 45 has plenty of bullets available, and, I think you may find it's cheaper to feed then the .44 magnum, due to the Cowboy shooting craze.
I would call a gunsmith that does conversions, like Linebaugh, Jack Huntington, Hamilton Bowen, and ask them what the numbers are they cut for tight .45 Colt chambers(You might be able to find this on their websites, maybe Linebaughs, but I think the information might be old). Then ask them what the average Ruger is cut to, in .45 Colt these days.
My understanding is the reason they start with .44's is they can bore the cylinder out, but, they can't put material back in, without making a new cylinder.
That said, the way Ruger cuts the cylinders may have changed as well. When
I looked into this, they started with the .44's, because it was pretty common that one of the cylinders in the 45 would be oversized. Also, it seems to me that the forcing cones are an issue in a tune up as well. The bullet going from an oversized throat, into an undersized cone. I'm not up on this, and, as I said, this stuff was years ago, and, I see parts of it resurface every once in awhile. Also, I believe as the bits get older, the cutting gets sloppier, or the bits increase in size, not sure on the last part.
As for the .475:
.475 Linebaugh
Reloading data:
No. 9 325 Jacketed FP 27.0 1,430 30.0 1,589 49,340 1.739
Recoil Energy of 29 foot pounds, and Recoil Velocity of 24 fps.
325’s
1544
1492
1492
1482
1519
325’ Are considerably quieter then the 275’s.
The 325's seem to burn the No.9 consistently, and in the 7.5" barrel I have. Hit of the three.
275 Speer HP grain bullet
1560 fps
28 grains of AA 9?
Recoil Energy of 25 foot pounds, and Recoil Velocity of 23 fps.
275’s
1441
1534
1596
I also have a stash of 400 grain Hornady factory stuff that chronographs at 1350 fps out of my 7.5" barrel.
This is my .500JRH, and, I like this barrel length:
5.5" is I think, the most comfortable compromise in barrel length.
The .475 is a custom by Jack Huntington, converting a .454 to .475. The 5.5" was a .475 Freedom Arms converted to .500JRH
Also by Jack Huntington.