I've done that splainin a few time. I worked on this for almost a year.
I bought 2 buckets of 223/556 cases years ago. They sat in the rain on the range a few days before being picked up.
(1) I ran them through the tumbler for 8 hours with walnut media, still black. I inspected them with a bore light for Berdan primed ones. De-capped the boxer primed ones, first use of a shell holder, they fit fine. Inspect case length, second time in shell holder, they fit fine. Trimmed everyone that was over 1.760 to 1.750, third time in shell holder, they fit fine.
I finished my wet tumbler at that time. I ran them through the tumbler for 2 hours. They were bright again. Ran them through the dryer tumbler for 45 minutes. Carried a can full to prime while keeping the granddaughters. 1 out of the first 10 fit the primer shell holder. So I put them away until I got home. I have 21 #4 shell holders and they wouldn't fit in any of them. I ask everyone for a fix, here, Lee, facebook groups, 3 other reloading sites and anyone I saw at a store in the reloading section. Most everyone said I needed a set of RCBS small base dies so I bought them, no fix. Some people said I needed to throw them away and buy more. I know there a fix somewhere and I'm not throwing away $60 worth of cases, at 4 cents each.
So that batch sat while I did another batch. They ended up the same as the first. So they sat and I asking again for a fix. Still the same replies and no fix. Third time into a facebook group a guy says a friend was having a problem with the cases fitting the shell holder. He removed the guts from his 38spl sizing/decapping die and used it as a bulge buster. I have Lee dies and it won't work. I ask what brand of dies he has. a few days later he replies they are lyman. Well I guess I'll look for a Lyman sizing die. Another person says he has some cases that won't fit the shell holder and he has a Lyman die. He'll try it before I buy. An hour later he replies, I removed the guts from the die and used the .357 cast bullet pusher to send them through the die, it works it's a fix. Expect to have to press extra hard on the handle, use an O frame.
I ordered a Lyman 38spl sizing die from ebay for $15. I removed the top cap and removed the guts. Pressed a case through and placed it in a #4 shell holder. Then another one. Finally a fix.