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The only reasonable answer I can think of would be if the bulge were too extreme, and that's a judgement call. .40 S&W runs at significantly higher pressure than .45 ACP hardball, so an equally weakened area in the brass is more at risk in it.
The funny thing is, the Redding GRx die is made specifically for dealing with Glock .40 S&W cases (GRx means Glock Prescription). So I'm not clear why Redding thinks this is OK to do this and Lee doesn't. I'd give Lee customer service a ring and ask. You could also call Redding and ask their experience with the life expectancy of such cases. I suppose it's possible the Redding die is shaped to do something the Lee Carbide FC die won't do to a case, but I have no idea what that would be?
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