From what you post, I understand the manual recommended WSPM- small pistol magnum primers for some loads and WSP- small pistol primers (standard) for other loads . Neither is a rifle primer - Winchester rifle primers would be WSR or WSRM. As I understand it, hand gun cartridges have a shallower primer pocket than rifle cartridges and only pistol primers, or mag pistol primer fit right. Rifle primers might not seat properly and protrude from the base of the case. I did it once by accident in a small batch of 41 mag - but I caught the mistake before charging the cases, and punched out the offending primers. I was being careless and didn't look closely at the primer packages.
I believe the priming charge in pistol primers is usually smaller than for rifles as usually a handgun cartridge has less powder to light than most rifle cartridges.
Pistol primers are often a little softer than rifle primers as they usually don't take as much pressure and hammer drop from a handgun may not be as strong as a rifle firing pin.
For some high intensity handgun cartridges, I think 357 MAX (don't confuse 357 MAX with 357 magnum), 454 Casul and similar - rifle primers may be needed - I'm going on memory here - check a reliable load manual.
For a while Winchester made only one primer for each size: pistol, rifle and they were designed to work with standard and magnum loads - I liked that system. I think they were relatively hot primers designed to ingnite the Winchester ball powders - ball powder loads very often call for magnum primers due to being a little harder to ignite.
As mentioned earlier, often mag primers are recommended for hard to ignite ball powders or large volumes of slower powders - like 296, H110. Standard primers are usually recommended for flake or fine grained powders in small to medium charges - powders like Bullseye, Red Dot, Unique, Herco and 2400.
For standard level 357 mag loads in a rifle or handgun I would stay with pistol primers according to the load recipes. I have 41 magnum rifle and handguns and follow this system. Barrel length has nothing to do with it.
I'd strongly suggest you get a full reloading manual like the Lyman, Speer, or Hornady and carefully read the chapters on reloading at the beginning of the book(s) - appears to me that you might find it helpful.
Be safe and have fun.