I knew it was a long shot but could one of you explain why it is a bad idea? To slow, to fast, can't load enough powder to pressure, etc...? Just trying to learn.
In general terms, it's a burn rate /pressure /case volume thing. Some powders are simply not suitable for some uses,
Consider this, I took a quick look through an old Lyman manual (1970s) which lists Ball C2 for many cartridges, and the smallest case where they used BLC2 was the .222 Remington, and the lowest charge was 23.5gr.
Every other round where that powder was used had charge weights in the 30+gr range, all the way up to 70gr in the .458 Win Mag.
It may be that BallC2 is simply horribly inefficient with too small a charge volume. I don't know for sure, but some powders are like that, below a certain volume, they do not burn correctly or completely.
Now, you could, maybe fit 23gr of BallC2 into a .45 Colt case, but then, IF IT WORKED, you COULD have the same kind of pressure you get from that amount of powder in a .222 Remington, and that is something you don't want to shoot out of ANY firearm made to fire the .45 Colt!!!
Too slow a powder usually means incomplete and erratic burning, and CAN do really strange things, including possibly detonating. And as far as I know, there is no way to tell what it will do, with absolute certainty. It may be a calculated 1 in 10,000 chance, but if the 3rd round you shoot happens to be that "lottery winner" then you are a loser.
I once got a couple pounds of 8700 powder in a trade deal, its a very slow powder, intended for big magnum rifle cases. I didn't have a big magnum rifle case, but I did fine ONE set of loads for that powder in the .45-70, so I thought, why not??
loaded 5 rounds, just to test, shot them from a Marlin 95. All bullets exited the barrel, but they were weak. Partially burned powder visibly streamed from cases as they were ejected, and the action filled up with "golden dirt".
(at that point, I was done with that idea..
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Can't (and won't) say Ball C2 would do this in a .45 Colt, I just don't know. I'm telling you this as an illustration of ONE of the things that can happen when the powder is too slow for the application chosen.
Sorry, but BL C2 is the wrong powder for .45 Colt.