I’m sure if you duplicated you 44spl load in 44mag brass you’d get a more consistent result. That’s been my experience with 357/38s.
So it’s a nice sunny day, and I don’t really need too much of an excuse to go shooting. So I loaded up some fast & slow loads in both .44 Spl & Mag cases as well as in .45 ACP & Colt cases. I shot the .44s through the Contender, and the .45s through the Blackhawk Convertible.
At the end of all of it, I’d say we were both right to some extent. You moreso about how the revolver behaved and me for the single-shot.
So I loaded up some 225 gr. FTX bullets in .45 cases. For my slow loads, I used a starting load of AA#5 in .45 Colt cases, and I found where someone had used Titegroup with these bullets in .45 ACP. My fast loads I used H110 with a near top-end load from the "Ruger Only" section of the Hornady 10th ed.
Average velocities were:
.45 Colt: 1268 fps.
.45 Colt: 719 fps.
.45 ACP: 696 fps.
In .44, I used a 240 gr. Zero JSP. Slow loads used Trail Boss for both Spl. and Mag brass. Fast loads were with a near-max load of AA#9.
Average velocities were:
.44 Mag: 1690 fps.
.44 Mag: 427 fps.
.44 Spl: 565 fps.
I shot at 25 yards from a rest for everything. The .45s on the left target and the .44s on the right. I fired 10 rounds of each load. The .45s generally all grouped in the same area, just a bit above point of aim. Shooting either fast or slow bullets from .45 Colt cases was a little more accurate than from ACP. I have found each cylinder with this revolver to have very bullet-specific preferences. Apparently neither cylinder likes this particular bullet. That gun generally groups better, but I usually load 250 and 300 gr. bullets. I just didn't think they would be as appropriate for the ACP case, and the FTX are rated for the full range of what the .45 ACP or Colt are capable of. I also didn't want to introduce another variable by using a different style/weight of bullet.
The full-velocity .44 Mag shot a couple inches below point of aim. And the much slower bullets fired from either Colt or ACP cases all hit about 10" higher. The length of the cartridge does not effect the POI, as I had noted previously.
I suppose the next thing to try would be to shoot both .44 Mag & Spl from a revolver with both fast & slow bullets to see if the case length makes a difference there. The test should also be valid shooting .45 Colt & .45 Schofield. But since I don't have any of the shorter cases, I likely won't get any just for this experiment.
It's been instructive, and good dialogue.
Hopefully this also answers some of the questions relating to shooting different cartridges/cylinders in the same revolver. Ultimately, the best answer is to shoot your gun(s) with the different cartridge/cylinder combinations and see how they perform.