Are right, stay with me – this may not be pretty! I am speaking as a guy that has loaded thousands of rounds of pistol ammo, loads of 38 Spcl, 9mm, 357 Mag, etc. etc. I say all this just to establish my bona fides, now down to the subject at hand - I have just become the proud possessor of a little Marlin 1894C chambered in 357, haven’t even shot it yet. In reading all the data I can find directed at such pistol cartridges used in carbines and rifles, I found an article with just that title in an old copy of the “Handloader” magazine. The author is pretty much in line with what I already know until I suddenly read: “Speer recommends the use of SMALL RIFLE PRIMERS in such calibers as the 32/20, 38/40, 44/40 and other such calibers when used in carbine or rifle.” Well, gag me with a spoon! Why? What is the advantage, what is gained? Does this mean if I load for the 357 Mag and shoot the rounds in the carbine, should I load them with small rifle primers vs small pistol primers? What happens now if I want to shoot some of these in a handgun?
I know the small rifle and small pistol primers are the same size and will readily interchange but what is the real difference? I always thought they were identical except for the cup of the SMP being of thicker, stronger parent metal than is the SPP {I tire of typing out ‘small rifle and small pistol]
So, if anyone has any knowledge of this self inflected dilemma I now find myself in, please let me know. Until then I intend to feed this little Marlin on a steady diet of 158 Gr XTP pushed with a healthy dose of 2400 and ignited with the same old CCI small pistol primers I have used for years in my handguns – it has always worked in them.

Do what the ammo manufactures do and load all the rounds as if they will be fired from a (old in there case "your" gun in your case)hand gun. That way if you deside to pick up your pistol and shoot it, you will not out shoot your cup/psi.
I do a lot of reading But been loading rifle for 3-4 years. I try to take all this in like a sponge! So I hope my .02 helps and if someone else can come in and help this is a dilemma I was looking to put my self into also.

Rifle primers have a tougher cup than do pistol primers. They can be used places where cartridge pressure may get a little high ... such as pistol cartridges used in longer barrels such as rifles where the powder is more likely to be completely burned in the bore, or hot loads such as one might develop in a competition .38 Super.

Have followed that rule for 40+ years and have not had any excitement due to barrel length. You're obviously welcome to do what you prefer.

Small pistol primers are easier to read for pressure. I have been doing some bullet testing in my 303 Brit and because I have some large pistol primers I used those with a small charge of H4227/AR2205. These were fired with the muzzle straight down so you can emagine the distance the primer flash has to travel to ignite the powder and it worked just fine. The reason for the recommendation of small rifle primers in rifle loads for 357 is rifle pressure - 47,000 CUP I think it is? You don't want to be firing those in a revolver. And that is too high for cast bullets anyway. Oh yes, a revolver will not develope the same chamber pressure due to the cylinder/barrel gap, and the free-bore that comes with it.

Rifle hammers/pins can easily blank a pistol primer.

I have an 1894C in .357 Mag, and have used both small rifle and small pistol primers (CCI) with good results.

Small rifle primers -- depending on brand and lot -- may have thicker or harder cups, thus allowing them to withstand higher pressures without piercing.

HOWEVER, even if this is true, if a round loaded to pistol pressures is used in a rifle, there is no mechanical reason why the load should require a different primer.

Using SR primers as opposed to SP primers theoretically allows you to load to pressures higher than permissible in a handgun, but still within the working range of a rifle. Otherwise, it makes no sense.

If you are loading for a rifle, usually you will be loading to a higher velocity then is recommended for a revolver. Pressures will peak getting the bullet moving (look at pressure curves) that is the instant of the high pressure. So; if you are loading for a revolver, peak pressures should not be any different in a given load. thus; a cartridge loaded for a revolver will produce that same peak pressure in a rifle that it produced in a revolver. That being the case a cartrige loaded for a revolver would be fine in a rifle, but a cartridge loaded for a rifle at higher pressure should require a rifle primer.

Oh poopy! If you are loading a high pressure rifle cartridge with pistol primers you always run the risk of pierced primers and gas in the face. Been there done that. roger