Managed to get out today and add some data to the thread using a S&W M15 .38 Spl and a Ruger GP100 .357. Only tested three loads side by side with SP and SR primers but these are my go to loads for these guns so figured if this works I can use up a stash of CCI-400's that I don't want to use in my .223.
All results are 5 shots at 10' instrumental using an Oehler 35. Air temp was 35 deg.
S&W Model 15-6, 6" barrel
Winchester brass, my cast 158 grain WFN powder coated bullet (Accurate mold 358160) 5.5 gr. of True Blue
............CCI-500..........CCI-400
High............904................920
Low.............889................898
ES................15..................22
Avg..............896................909
Ruger GP-100 6" barrel
R-P brass, same 158 WFN cast and powder coated, 7.0 gr. True Blue
............CCI-500..........CCI-400
High..........1091..............1096
Low...........1049..............1030
ES...............42..................66
Avg...........1068..............1078
R-P brass, LBT 160 WFNGC, 16.0 gr. 296
............CCI-550..........CCI-400 - Note the small pistol magnum primer here, that's what would normally be used with 296 powder
High..........1318..............1348
Low...........1265..............1216
ES...............53.................132 (yowza!)
Avg...........1301..............1284
Groups for each load were not significantly different between the primers in all three examples and are about what might be expected firing two groups of the same load side by side. In fact this last load with 16.0 296 and a huge ES produced the tightest group of the day and the .38 load with CCI-400's was the next tightest. I have no way to measure pressure but brass extracted normally and obviously velocities stayed pretty close.
Basically this shows the same as Waders and Cheyenne - there really isn't any significant difference between using Small Pistol and Small Rifle primers although it does appear that the SR do provide a slight increase in velocity.
A final comment on the last load using 16.0 296 and CCI-400. The individual velocities in the string are 1216, 1292, 1288, 1348 and 1280. It looks like it wants to cluster around 1280-1290 fps with one very low and one very high reading, that skewed the ES badly and lowered the average so it was less than with the Small Pistol magnum primers. Or maybe that difference is true due to the SPM being hotter than standard SR primers. Don't have any ideas on that, as noted it produced the tightest group of the day at 25 yards. I might have to repeat that comparison with a couple of 10 shot strings.
