Tuzo said:
Since the .45 ACP case is longer than the .455 case the extra pressure should be evenly distributed over a larger area, hence safely workable.
Not so. You're thinking of a fixed amount of force being distributed over a wider area, thereby "diluting" the pressure it applies to each square inch. Pressure itself already takes distribution into account by being the same amount of force per unit area, regardless of how big that area is. At a given pressure, doubling the area doubles the total amount of force being applied.
The problems with 45 Auto data are that they are taken with a 5" pistol barrel, where the Mk. IV has a 6" revolver barrel, which is about 7.27" when measured as a pistol barrel is, though you've also got some barrel/cylinder gap venting gas plus an undersized bore, so it's difficult to get a real equivalent velocity or pressure number.
Currently, the CIP lists 45 Auto Maximum Average Pressure (MAP) as 1300 bar and 455 MkII MAP as 900 bar. While there is a lot of absolute number disagreement among cartridge pressure measuring systems, since those two numbers in bars are from the same measuring system, it is a pretty good estimate that you need to multiply the 45 Auto MAP in any other measuring system by 900/1300 or 0.692 to get the 455 Mk II MAP for that system.
For SAAMI units of CUP:
18,000 CUP × 0.692 = 12,461 CUP MAP for 455 MkII
21,000 psi × 0.692 = 14,538 psi MAP for 455 MkII
Quickload shows that for Hodgdon Universal, you multiply the 45 Auto maximum load by 0.815 to bring the peak down to 455 MkII level. This number worked for Bullseye, too, when I tried it. I had to use 0.800 with Clays. So this varies a bit by powder, but not dramatically until you get to powders slower than you'd want to use, like Accurate No.9, for which you only need 0.92 as a charge weight multiplier.
In your shoes, I'd be tempted to get a custom mold made to drop bullets that fit the chamber throats within half a thousandth.
Cjwils,
After Alliant issued the
warning not to use blue dot in 41 Magnum at all and not to use it in 357 Magnum with 125-grain bullets, I am reluctant to use it in untested loads at all. Alliant has never explained the issue, so we can't tell where it applies and where it doesn't.
