So both my hornady 7th and lyman cast call for SPM primers on the 357 mag and SP on the 38sp.
My question is if you substitute SP when a SPM is called for can you have ignition problems that could cause dangerouse presure spikes. For lack of a correct term a double preasure spike?
My lack of trying to relay this so please be patient
If you have a load that calls for a SPM primer and you use a SP. Could this cause a incomplete combustion of the power effectively leaving a chunk of the un burned powder to compress behind the bullet only to have a slight second ignition of the remaining powder once preasure builds? On the idea of the rifle loads that use fillers and result in " chamber ringing" ?
could this be even worse when using a dense powder that doesnt fill the case ?
Not with a revolver. With slow pistol powders and a revolver, if the manual calls for a SPM primer and instead you use a SP, you will get inconsistent ignition, inconsistent pressure, inconsistent velocity, and occasionally even squibs.
To get a complete consistent burn, you need a strong crimp, a charge weight above the minimum, and if the load calls for it, a magnum primer. The powders that do best with magnum primers include 2400, W296, H110, Lil Gun, IMR-4227, and sometimes AA9. Longer skinnier cartridges are more likely to need a magnum primer than a shorter, fat cartridge. In other words, you are more likely to need a magnum primer with the .357 and .460 Magnums than you would be with the .44 or .500 Magnum.
ETA: These powders need pressure and heat to undergo a complete burn. You have to have both - without sufficient initial pressure, the temperature stays too low. A strong crimp along with a sufficient length of the bullet inside the case combine to retain the bullet in the case an instant longer than a weak crimp and shallow seating depth, which allows the pressure to build up to the proper level. A magnum primer jump starts the process with a more explosive spark.
The reason why a magnum revolver is different is that as soon as the bullet clears the cylinder and uncovers the cylinder gap, the pressure drops. If everything isn't fully rolling by then, your ammo will suck. It won't develop unsafe pressure, it will just be very inconsistent.