Another Winchester vs CCI SPP question for .357 Magnum loads

[Six]

Last year I started loading .357 Magnum with Winchester SPP and Unique and 2400 powder.

Nearly in every load/bullet combination, flattened (though not flat) primers started to show up at 90-95% of max load and I backed down to 90% and all was well.

Recently I finally got a hold of some CCI SPP and loaded the same loads up, same powder, same bullets, same OAL etc, and I've yet to see signs of primer flattening.

So, are Winchester primers hotter resulting in higher pressure, are CCI primers more resistant to deformation, both, or other explanation?

 

[The Bushmaster]

Do not equate flattened primers with over pressure. It is just not so. The reason that the CCI primers didn't flatten as much is that the primer cup is a bit harder or thicker metal. I run 2400 and 140 grain SJHP Remington bullets to their limit with no problems. I run W-296 to with in 95% of it's top limit and no over pressure signs.

What is happening when you fire your revolver is that the hammer shoves the round forward in the cylinder just before it sets off the primer. The round fires and the primer backs out a small amount. The round is forced back against the recoil plate and the primer is reseated and sometimes flattened in the process.

This may be different with different firearms as when using W-231 in my S&W mod 19 I have to stay around 80% of maximum load or I start getting sticky extraction from the cylinder.

 

[Steve C]

Winchester small pistol primers are a bit warmer than CCI in my experience. I've chrono'd the same loads on the same day but using the different primers and found the Winchesters yielded higher velocities in .38 spl loads. Winchester magnum primers are very hot, probably the hottest magnum SPP on the market.

CCI's are slightly harder primers and some of the difference you see may be due to their construction rather than a significant change in pressure. As with any component change when reloading there can and usually are changes in performance to some degree.

 

[Six]

Do not equate flattened primers with over pressure. It is just not so.

Sure, but, flattened primers in my loads very closely corresponded to increased powder charges. And as I could clearly see that primers got flatter and flatter, I thought it might be wise to back off some. It might not indicate over pressure, but it did indicate that pressure was rapidly rising.

It sounds like the difference lies in both pressure generated by the primer, and that the CCI primers have a somewhat harder cup. Interesting, I'd assumed that primers were pretty much equal.

Maybe in both cases pressure is at perfectly safe levels, I just know that my comfort level is suddenly higher with the CCI primers

I appreciate the responses by the way.

 

[The Bushmaster]

Six...One of my three .357 magnums is an older Ruger Blackhawk. The other two are a Colt SAA and a S&W Combat magnum.

As long as the fired cases are easy to extract you are probably well within safe pressures. If you have to exert any effort to extract a case it was over pressure. Another thing to watch for is blown primers and soot around the primers. These ARE indicators of over pressure. A good test of this is to see if you can extract a case with your finger nail...If you can't...Well.....