I'm the opposite of Hawkmoon as I practically never shoot jacketed or plated bullets in the 1911 - only cast lead. But that doesn't keep me from experimenting with Hawkmoon's kind!
Another reason why the same weight lead and jacketed/plated (J/P) bullets can behave differently (in pressure and/or muzzle velocity) is because of their diameters. Lead bullets can be anywhere from 0.001" to 0.002" bigger and because of that do a better job of filling the barrel grooves. I've measure a lot of commercial J/P bullets and have never found one measuring 0.4520"D. Most were in the 0.4505"D to 0.4515"D range. Because of this, J/P bullets generally have a lot of blow-by when fired, as shown below,....
Mythbusters 1911 HS Video Snapa.jpg
Of course barrel groove diameter is also a factor in blow-by, pressure and velocity. But logic tells me that if a lead bullet fills the grooves better and thus has less blow-by, then with the same seating depth/residual case volume, bullet weight, powder and powder charge, the lead bullet ought to have a higher pressure and muzzle velocity. In addition, the max load for a lead bullet should then be less than that for a J/P bullet.
But that's just my logic, which may have little or nothing to do with the matter at hand!
When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind. [Lord Kelvin]
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