pressure in jacketed vs lead bullets

[dgang]

 :confused: 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

[ellis4538]

Type in Allient Reloading Powders and when you link to their web site find "contact us" and ask them.  They should be able to give you the technical details so that you can understand them.

[cookie62]

I asked someone that question one time and the answer they gave me was because when using lead the bullet expands and fits tighter in the lands and grooves causing higher pressure. Don't know if this is correct or not, bit that is what I was told.