The Lee manual is a good one, but unfortunatly they leave a lot of information out. And since it's the cheapest one available it's the only one people think they should buy.
The only factory .45 AR round available was a lead RN available from Remington. It was pretty soft and would lead barrels at higher velocities so it was loaded "light". Since this was the only factory load, the SAAMI specifications for pressure were based off of that one load. Internally the AR brass is the same as ACP brass and loaded to the same data as ACP will give you the same pressures.
It's hard to tell what the statement in the Lee manual means because they don't explain it. It may mean that they didn't work the loads up and just reduced them from .45ACP data. What it does tell me is you can't own too many reloading manuals.
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