The standard primer vs. magnum primers with #2400 powder debate will likely go on forever. I was curious about this several years ago. I loaded 12 grains #2400 with the original .357 Magnum bullet - the Hensley & Gibbs #51 plainbase 158 grain SWC cast from wheelweight alloy. I fired five, five-shot groups at 25 yards from a benchrest using the same load, only switching primers between each 25 rounds. I used a Colt Pythom with a six-inch barrel. Loads were chronographed and all group sizes were measured.

Primers: CCI -standard & magnum, Federal- match & magnum, Remington- standard & magnum, Winchester- standard and magnum.

40 groups of 5 rounds each; 200 rounds total.

Muzzle velocities varied from a low of 1183 (Federal small pistol magnum) to a high of 1261 (Win. small pistol magnum). Exactly the same primers gave best accuracy (1.95" for Fed. magnum) and worst accuracy ( 3.10" for Win. magnum). All other primers provided very similar groups sizes, ranging from 2.25" to 2.75".

With the exception of the two extremes, performance was very close in all respects. There was a very slight accuracy advantage with magnum primers, but not enough to convince me to stop using standard primers. However, long-range handgunning, something few of us do, would probably make me reconsider.