Gloob, would you mind answering the rest of the questions I asked in Post 22? I had some pretty specific questions and Murf didn't cover those answers for you.
Ok, here goes.
First off, now I see a little better your standpoint. If you only load 45ACP, then you will, indeed, see a very little difference between max lead and jacketed data.
In a locked breech pistol, minimum charge weights are limited by function of what will cycle the gun. Maximum charge weights are limited by peak pressure or maximum slide velocity, one of which may occur before the other, depending on the cartridge and the platform and the specific components and powder involved.
If you only load for 45ACP, you will, indeed, find very little difference between maximum cast and jacketed load info. It's not because 45 caliber bullets work the same, whether cast or jacketed. Or maybe this IS the case to a degree, seeing as the bearing surface on a wide 45ACP bullet is relatively smaller than on a 9 or a 40. But it's also because of the platform. 1911's can handle it. Most 45's can handle it. So the fact you can use maximum jacketed data for a lead slug and still be ok is specific to this caliber because of the guns that you are shooting it in. When you substitute max jacketed data with the cast bullet, you are getting higher bullet velocity and higher slide velocity, but that's fine, because the gun still works, 99% of the time. You will get higher recoil, though. Think of the max cast data as +P, even though the peak pressure isn't beyond SAAMI.
If you look at, say, 9mm cast vs jacketed data, you will see there is, indeed, a much bigger difference in the max loadings. Just look in any old loading book that has info for both, and you will see that 45ACP is an anomaly in this regard. I hypothesize this is because of 1. the relatively longer bearing surface on these bullets, and 2. that 9mm/40 SW handguns are more often limited by the max slide velocity, rather than peak pressure. Operating at the higher pressures they do, 9mm/40SW brass is thicker. Chambers have better support. And there are a heck of a lot of people shooting smaller 9mm, like the Browning HP or even smaller guns. When you substitute max jacketed data for a cast bullet, the peak pressure is not a problem. It's the slide velocity that is the problem. That slide velocity will batter a HP. And in some firearms, the increased slide velocity will increase the incident of failure to extract.
Extraction occurs shorty after the bullet leaves the barrel. The brass "sticks" to the chamber walls for a short while after the bullet exits. If the barrel unlocks and tries to extract too soon, the spent case will still be stuck, and the brass will remain in the chamber.
Published load data isn't going to get you into these fringe/extreme regions where slides don't cycle or some damage might occur.
In a 1911, I don't think you can get into this danger zone. In a G21, I don't think you can get there. But in some other firearms, I think you can get there. I used the example of a Glock 27, because I think I have been there, myself. Using a book max jacketed charge, I substituted a Rainer plated TC bullet for the Berries TC, same bullet weight. Out of my FNX, it felt totally normal and accuracy was fine. It was too much for the G27, and it caused failures to extract. Bore was clean. Brass was not bulged. In fact, Rainier's suggests to use cast load data for their plated bullets. Berry's suggests to use start to midrange jacketed data. The construction of the bullets is a little different, and the plating on the Berry's is thicker. Maybe those manufacturers actually based those recommendations on something other than a wild guess?
I have also had a failure to extract in a G19 using 124 gr cast over 5.0 gr Unique. Again, bore clean, brass not bulged. I have shot tons of Berry's 124 gr plated bullets at 5.4 and 5.7gr UN without a single malfunction. I backed my cast load to 4.7 UN and have shot plenty more without issue. 500+ rounds with 100% reliability.
Some guns just don't function reliably if you go beyond what bullet momentum and slide velocity factory ammo can produce.
Aside from difference in friction down the bore, I think maybe the larger cast bullet take a little more forcing into the leade? I dunno. I think this might be part of the reason that it takes less powder to get the same velocities. It's like having a tighter crimp on the case, built in. I think this is part of the reason my G27 had such a big problem where my FNX didn't. Glocks are known for having abrupt leades. Any rate, for w/e reasons, the cast bullet goes faster with the same charge. And another difference I found between these two guns, is that my FNX has a heavier slide and it stays locked for quite a lot longer than my G27, which I discovered after taking calipers to it and examining how and where the two guns unlock and start to extract. So, I deduce that my G27 has a lower slide velocity limit than my FNX, to where extraction can become problematic.
Consider some firearms rated for "limited +P" ammo. So what happens when you fire too much of it? Does the chamber eventually crack due to the pressure? No. If that were the case, the gun wouldn't be rated for any SAAMI standard ammo. The chamber and barrel has to withstand at least a 50% overpressure for any manufacturer with a brain to consider putting it on the market. What happens is the recoil spring gets shot in a relatively few number of rounds, and the frame rails take a battering, pins bend, and the gun generally just wears itself loose. The frame or slide may even crack. Gun-folks seem to think peak pressure is the key to everything. In a bolt action rifle, that may be the case. But consider what actually wears out a locked breech handgun and causes malfunctions; it's slide velocity. When you convert a 1911 for 460 Rowland, do you put in a stronger, thicker chamber? No, you put in a stronger spring and a muzzle brake to slow the slide. When peak pressure is the problem, you will know because your gun will explode. And that will occur when you double-charge a round, causing the brass or even the chamber to fail. Or in the least, your brass will be bulged out in the case web. If you're a careful reloader, you will hopefully encounter slide velocity problems well before pressure problems in a locked breech gun. But again, it depends on the exact design of the gun. In a gun with crappy chamber support and/or crappy brass, pressure may be the limiting factor - ala Armscor 10mm in a Delta Elite. If you're loading for that combination, you might very well be able to load to higher velocities using cast bullets vs jacketed.
At any rate, there is cast data and there is jacketed data. They are different for a reason. Interchange them at your own risk.
If you feel like there's a specific question I have not answered or that I have skirted, or that I have twisted your words, then restate your question, and you shall receive a response. But I will no longer attempt to explain the laws of physics on this forum. BTDT, and I can in some small way sympathize with Galileo's persecution by the Catholic church.
