from the old forum:
Does ignition influence secondary spikes? Here's a repeat of the March 24 cast load with Ramshot Big Game and a 180 gr. cast bore rider, except today we used a CCI #250 magnum primer. There has been no evidence that the secondary spikes are caused by poor ignition, but just to be sure I tried the magnum primers to see if they would make the spike go away. Obviously, the spike is still there. ES was only 5 fps, so Big Game does seem to like the magnum primer, spike or no spike.
Here's the same load except with a crimp and Fed #210 primers. The crimp is another test to see if ignition influences the secondary spike -- obviously not. Velocity was 2621 compared to 2664 with the magnum primer.
As I see it, the secondary spikes correlate to slippery bullets, and to certain powders, but not to crimp or to primer.