Thanks for your patience and one final question...Seating a158gr 38 cal RNFP xtreme into cannelure mearures coal of 1.411". The xtreme data says 1.440". So this isn't excessively short coal? My Lyman manual warns about excessively short coal/over pressure but doesn't define it. I usually shoot my 38spl loads in +P Model 10/64 but to be safe I could shoot this particular recipe in a Ruger GP100.
.030 isn't that much considering the capacity of the case and the relatively low pressures you're running, but if you are concerned that much, don't load them shorter to the cannelure, load them to 1.440. The different brass isn't the issue as far as the cannelure reaching the case mouth.
You're worried about loading them shorter, but not about loading a plated bullet in place of a jacketed bullet, knowing that will affect pressure as well, which you have been doing all along.
That said, I am still a little confused because you go back and forth from "Hodgdon XTP" data to "X-Treme data" etc. I don't see any X-Treme W-231/HP-38 data in their PDF.
Substituting a plated for a jacketed bullet can change pressure, up or down. Changing OALs lengths obviously changes pressure. Longer less, shorter more (That one's easy).
W-231 isn't the best choice for +P in .38 Spl IMHO, something slower is better suited. I don't see any W-231/HP-38 data in the X-Treme PDF, so why are you concerned with their OAL shown there? Hodgdon .38 Spl +P online data shows a max of 4.6 Grs HP-38 and a 158 Gr XTP @ 1.455 OAL, which is even longer. 807 FPS.
I would suggest we have no idea what your pressure is on the ones you have been shooting, or will be with the different X-Treme bullet @ 1.140, 1.440, or 1.455 OALs.
The good news is .38 Spl pressures are low, giving you a little wiggle room in strong revolvers like your GP-100, but that doesn't give us carte blanche to go heavier on the data.
While you're probably "OK", did you work up the last load to 4.3 Grs? Do you know what those velocities were?
I load the X-Treme 158 Gr to the "canellure" (It's really just roll marks), but you don't have to. I worked up my loads with that bullet at that OAL, starting low. I have loaded it up to near max .357 velocities in .357 Mag brass, but fire most of them in light plinker loads in .357 Mag brass. My plinker load is in no manuals, but I am confident it is low pressure, and I use it in .357 revolvers. I have used it in plinker loads for .38 Spl, but prefer the Berrys 148 Gr HBWC for that (Gives better PF & PL numbers by reducing empty case space), although the X-Treme 148 Gr DEWC also shots well.
I checked my notes and I loaded the X-Treme 158 Gr SWC to 1.440 OAL when I tried it in .38 Spl. But it doesn't really matter if you load it to the cannelure or not and lightly roll or taper crimp. Like I said, it's just roll marks, not a real recessed cannelure to crimp into.