Re: Reloading for the .357 Magnum
Use the primers called out in the reloading recipe in the manual you got the load data from. If it isn't in a manual (or in the manufacturer's online load data) then don't use the modified recipe. Use the recipe as listed there.
Most reloading manuals have reduced loads data for approaching the 38 special levels. Again, find the load you want in a manual and use it exactly as shown. Arbitrarily lowering the load level from a manual below the starting load CAN be dangerous as too much air space in a cartridge can be as bad or worse than too much powder (detonation).
Also the more air space in the cartridge the more varible the ignition can be dependent on how the powder lays in the cartridge in the gun when it is fired. Huge difference can occur in the pressures from the powder being all the way to the rear to all the way forward. The most repeatable loads often are the ones that are closer to filling the case, unless the powder has been designed to work equally well where ever it is inside the cartridge. Powders like W231 or Hodgdon HP38 or other fast powders that use small quantities are usually designed for small quantities of powder in the case whereas Unique and others MAY not be.
So, follow the published recipes exactly as shown to stay safe.