pressure in lead vs jacketed bullets

[dgang]

I recently started to load again for my .357 and I am confused about listed pressures. For example, using Alliant Unique powder a 158 gr. jacketed bullet can be loaded with 7.8 gr. of powder resulting in a velocity of 1280 fps and 33,200 psi. A lead bullet of the same weight can be loaded with 6.8 gr. of the same powder ( 12% reduction) with the same OAL resulting in 1295 fps but at a higher pressure of 33,900 psi. I understand that the lead bullet creates less friction so less powder can be used to obtain the same velocity, but I can't understand the increase in pressure using lead bullets and less powder.Anybody out ther have an explaination for this?
Thanks in advance, dgang

 

[Slamfire]

I don’t know if there really is an “answer” as much as there are theories.

My theory: Ever had a patch get lodged in a barrel? A twisted, jammed wet patch can be a very hard thing to knock out. You push on it and it changes shape and clings to the barrel walls. The way I think of it, pushing lead bullets down a barrel is similar to trying to knock out a jammed patch. My best guess is that pure lead bullets deform more than jacketed bullets in the barrel. So a lead bullet actually is harder to push out the barrel.

That is up to the point that you vaporise the lead. If you get gas cutting, then the lead plug is smaller than the barrel and the pressure would drop.

 

[Steve C]

The explanation that makes sense to me is that lead bullets are slightly larger in diameter than jacketed and being softer they seal the chamber and bore better which results in greater pressure than jacketed.

 

[Bullet]

Steve C
I think you are correct. I asked about this at Hodgdon and was told what you stated.

 

[Walkalong]

I generally takes less powder to get the same velocities with lead because it is slicker, with help from the lube, than jacketed, despite the fact that it "slugs up" easier than jacketed.