I can see where volume would be the most accurate comparison, since rim thickness can vary and weight won't tell you if you have thicker walls or thicker rims.
Makes sense to me.
I actually ordered a couple of .38/357 molds today. A Lee 158 FN and a Lyman Devastator HP.
I know, I know, the Lyman is advertised as a .356" 124 grain 9mm/.38 Super mold.
Thing is, my buddy has this mold and all of our bullets cast from it dropped from the mold at .358" and 128 grains.
So, we can size them down for 9mm, or lube/shoot as cast for .38/light .357.
Double duty.
If we need, I can make a top punch and bump them up a thou or two.
Everyone should own a couple of lathes, mills, and shapers.
Anyhoo, I've never given much thought to case thickness variances between .38 and .38+P.
I just bought them and shoot them. I'm not new to reloading but I am new to reloading .38/.357.
I tell everyone I'm going to load .38/44 ammo for my .38s, but mostly I'll load mouse-fart .38 and .357 target loads, and a few .38/44s and real .357 mags for hunting use.
I once shot a box of Hirtenberger 100 grain SP 9mm+P+ out of my FEG P35 clone.
It was anticlimactic, to be sure. It recoiled like my Ruger MkII .22 LR.
Not what I expected.
Bet there isn't any real difference between standard 9mm and 9mm+P/+P+ brass, either.
Dang, my posts have a tendency to ramble.
I need to work on that.