Everyone is making more of this that there needs to be! All of you need to compare loads for the same bullet weight, cast and jacketed, in the same manual(s)! Often you will see that Lead bullet loads are actually slightly higher than those for Jacketed in the same manual. So much for "Load plated with Lead data!"
For practical purposes both plated and cast bullets can be loaded with jacketed data if nothing else is available. The real difference that causes different data for different bullet types is seating depth, the distance a bullet intrudes into the cartridge case. Seating depth, on bullets with cannellures, is easily determined by measuring the distance from the bullet base to the point in the cannellure you will be seating to. Older manuals (Pre-WWII) usually listed seating depth, not over-all length, but this is one place where loading information and practice really has been "dumbed down" because it is directly measurable on a loaded cartridge case. Bluntly, the loading manual publishers think the average re-loader is too stupid to understand the "Seating Depth" concept!
The reason "Jacketed data" is often lower than that for cast bullets, of the same style and weight, is because the jacketed bullet is longer for it's weight as the average density is lower. The jacketed bullet is longer and seats farther into the case, reducing the powder space and increasing pressure as a result. The charge is slightly reduced to off-set the deeper seating depth and keep the pressure within limits.
As far as keeping velocity below 1300 FPS, or even 1200 FPS, this is sort of a joke really as few handgun cartridges will exceed this velocity level in and normal barrel length! So why does it always get mentioned??????
