Typical pressure reduction from "Js" to boolits?

[Steven66]

Is there lab data/a rule of thumb/accepted percentage for the pressure differential between a jacketed bullet and a cast boolit loaded with the same powder charge? I expect the general size of the cartridge is part of it.

[Larry Gibson]

None that I am aware of......too many variables........

[yeahbub]

I'm with Larry G on this. In .45 ACP I worked up a max load for the 200gr XTP, but I had to stay well back of that max when substituting the LEE 200gr TL SWC. I believe it had to do with increased drag due to the softer boolit's obturation. In .30-30, a cast 150gr Loverin (311466) is definitely easier to propel than a 150gr jacketed. Generally, cast are easier, but it's not to be taken for granted that a cast boolit substitution will automatically produce lower pressures.

[Landy88]

On another forum, John Barsness answered this question; that in powder company testing, to which he was a consultant, cast bullets have had a higher pressure peak than jacketed. Cast obturated quicker and better, for a better seal.

Much loading data bears this out with maximum loads for cast averaging a bit less than for jacketed, even though the cast often achieves slightly greater velocities. More for less, just another cast bonus.