A member of another forum contacted CCI technical during the shortage of 4 +/- years ago and this is the exchange:
"I called not to complain about availability, but rather to
discuss the technical differences between their primer types. I was put in
touch with one of their tech reps who was very happy to discuss their primers.
BTW, She mentioned that she had worked there 38 yrs and never seen anything
even close to this kind of demand.
Then she said that they can produce 3 million primer per day
flat out (which they are currently running) but have currently over 1 Billion
back ordered. Unfortunately, that means that if all new orders stopped
completely today. Everything; ammo, gov't, and component, it would take almost
a year running flat out to catch up.
Anyway, I was asking about the cup thickness, formula
differences, and formula amount differences between their #500 (SP), #550
(SPM), and #400 (SR). She had me hold a minute to get the detail specs up on
her screen and this is what she said.
Cup thickness: The #500 has a thinner cup than either the
#550 or #400, however, both the #550 and #400 have the same cup dimensions
(including thickness).
Flash powder formula: All three sizes use the same formula
for the flash powder.
Flash powder amount: The #500 has a slightly smaller amount
(3 micrograms) than the #550 or #400 which both have the same amount.
I asked if the SR primers could be used as an acceptable
substitute for the SPM primers. She compared the #550 and #400 and then replied
that yes, they appeared to have the same specs, same dimensions, same cup
thickness, same formula, and same amount of flash powder. She even noted that
the SPM primers were slightly taller than the SP primers and were spec'd the
same dimensions as the #400.
I asked if she knew any reason not to just use SR primers
for both magnum pistol and rifle applications based upon that information and
she said that many there only bought rifle primers and used them for all their
reloading, magnum or not. The only exception being for custom pistols where the
thicker rifle cup contributed to misfires, which she said only occurred in
custom race pistols."