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Just to add to the information, and isn't probably relevant in this case, I also get case head separation with 45-70 cases when using the gallery load. Five grains of 3f powder with a 0.457" ball seated directly on top of the powder in the case. Three quarters of the case now becomes the chamber and as the ball passes down the inside of the case it stretches it and after 6 or seven firings they start to let go. I use old cases that have been retired from full load use so I get a lot of mileage out of them this way.

To check for incipient case head separation I use a length of piano wire with 1/8" of one end bent at right angles, pull this "feeler" slowly up inside the case from the bottom and you will feel the ring that becomes present when the case is going to fail.

Help you differentiate 'tween a separation ring and a sizing ring on the outside of the case.

Sorry for delay in replying - been a good shooting weekend :)

To answer some of the questions asked above.....
Averaged over 20 rounds, these loads chrono at 1306fps. Looking at my Lyman 50th edition for 357 Rifle data for the relevant bullet, the starting load suggestions have MV ranging from 1257 to 1628 and the max load from 1491 to 1785.
Cases haven't been annealed, were bought new and have only been reloaded twice. Not been trimmed - personally I wouldn't expect to with a pistol calibre after only 2 firings!
Never fired 38 Specials in it.

Like the idea of feeling for the ring internally. Will try that soon.

Thanks for all the ideas.

What is the bullet tension?

What bullet are you using?

dromia wrote:What is the bullet tension?
What bullet are you using?

158gr RNFP - Just enough crimp so they don't push back in the magazine.

What hardness are they, are they commercial or home cast? and what is the bullet tension, difference 'tween the sized case mouth inside diameter and the bullet diameter?

What are the chances that the case isn't fully engaged into the chamber before she firing pin strikes? No under lever safety might cause this???
Seen it in .22RF.

Is that not headspace?

dromia wrote:What hardness are they, are they commercial or home cast? and what is the bullet tension, difference 'tween the sized case mouth inside diameter and the bullet diameter?

Commercial ACE bullets (ACHC38158S). Says "GM Hardcast bullets" on the box. Inside of sized case is .355 and bullet measures .358

Do you neck expand before seating?

dromia wrote:Do you neck expand before seating?

Just the usual Lee expander die to get just enough flare to get the bullet started.