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Guy,
If you are referring to the listings in say a Lyman manual or load data using factory made swaged lead bullets, that has less to do with pressure to the extent it has more to do with leading and the bullet stripping in the bore.
Hardness variables in lead alloy bullets actually allow for a wide range of pressures and loading levels.
You have the hardest lead alloys that can be made to move out as fast as the top loads with jacketed bullets.
The softer the alloy the lower the pressures it can take upon firing as a rule of thumb. The bullet may lead the bore, deform to the point of raising pressures like a plug or actually strip in the rifling when it is kicked in the rear by a higher pressure loading.
In rifle loadings you have two schools of thought, some use faster pistol powders for the lighter/slower velocity loadings and you have others that use the medium to slower burning powders for heavier/faster loadings with more gradual pressure curves. An easier "push" if you will. This will result in giving the cast bullet a chance to engage the rifling easier without stripping.
The use of gas checks, better bullet lube and using a harder alloy sized properly to the gun, can result in a lower pressure loading behind a cast bullet with performance alongside the jacketed counterparts.
As a matter of fact, in say a handgun, hard cast gas checked bullets in equal weights as jacketed can result in lower pressures and faster velocities with similar loads.
With lead you have a bit of a balancing act between bullet hardness, pressure, lube strength and sizing going on. This can be controlled by the handloader/caster to his advantage. Something not possible with jacketed.
Cast bullet hardness can result in a rise of pressures for the particular loading but there are usually other variables at work too.
This can get to be a very involved, vast subject rife with theory and argument but I've just touched on some salient points here for now.
FWIW
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