Try and look at primers without the "magnum" name. For this discussion, let's call 'em HOT and regular. The reason I say this is that people confuse "magnum" with their cartridge.
A .357 "Magnum" doesn't necessarily require a "magnum" primer. It's the load data and more specifically the powder that demands the HOT primer. A HOT primer has a thicker cup and burns a bit longer with more intensity. The purpose of the HOT primer is to reliably ignite certain powders that are notoriously hard to evenly and properly start. Perhaps the finest example of such a powder is H-110 or W296. These powders always require a HOT primer.
I make a slew of .357 Magnum loads and I haven't once yet EVER used a HOT or magnum primer at my bench. Quite literally, I've experiment with easily more than a dozen and a half different .357 Magnum loads in many bullets weights and with a lot of different powders. Still have never bought nor ever primed any case with a magnum primer. Never say never, but I don't intend to.
Many folks have stated that the evidence suggests that a small rifle primer and a small pistol magnum primer are the same primer. Of this I cannot be sure, but IIRC, someone in the industry stated that this was fact with regards to his company's primers.
One thing that jumps out to me from your post is that you intend to load .327 Federal Magnum in the future. For this round, do absolutely use a small rifle primer in your loads. The original factory ammo all made by Federal/Speer (ATK) is and has always been loaded with small rifle primers. It's my suggestion that no matter WHAT the load data says from any source, work your .327 Federal loads with small rifle primers.
Why that matters: rifle primers have a thicker primer cup and that cup contains the pressure in the round as it attempts to go anywhere to escape. Using small pistol primers in high-pressure loads can lead to pierced and leaking primers. Firearm and personal damage can result. .327 Federal runs a maximum pressure that's significantly higher than most typical handgun rounds that we are familiar with.
Last primer tip for this post: all primers are NOT created equal. Primers from different manufacturers will absolutely act differently in your loads. Where this matters to you is that if you are working at starting level loads, try whatever you like -- even "magnum" primers. But when you start advancing a load and taking it toward the redline -- NEVER just start working with a new primer. Re-work the load from a safe point. If you take a load you've developed and it's a HOT one and you run out of XXX brand primers and simply buy a box of YYY primers and keep loading those HOT loads, that's where you set yourself for failure.
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.