[onespeedbiker]

Okay, I'm confused. The only difference between a 38 spl case and 357 case is the 357 case is 1/8" longer, so it won't fit in a 38 Spl cylinder. Assuming you have not resized the neck, a fired 38spl/357 mag internal lip diameter (ID) would be around .357 (the bullet diameter for both 357 and 38 spl) which jives with what you said However, if the case is necked down to .357, that would seem to mean the outside diameter (OD) below the shoulder is larger than a 38 spl/ 357 mag case, yet it fits in your 38 spl. When you say, "It also is exactly at the length of a 38 special", are you talking about the entire case or the length to the shoulder. You may mean that it is a 357 mag case with a reduced case mouth so it would fit inside the headspace stop of a 38 spl, but that would require the bullet to be smaller than 357 as the stop inside the cylinder has internal dia of 357.

OTOH if the case mouth of a 38spl is reduced to 30 cal, then it is a 30-357 (aka 30/357 Paxton), or less likely the 300 Max Cat, which is a necked down (.30) 357 max case . Both utilize .30 rifle dies to neck down the case. Check it out; https://dandtcustomgunworks.websitetoolbox.com/post/300-max-cat-30357-5744675

 

[onespeedbiker]

But that doesn't account for the reduced rim thickness with the headstamp filed off.

True, but it may be something as simple as the way the barrel chamber was drilled, had a slightly different shoulder angle than the ammo being produced, so the rim was filed down, or the shoulder was formed too high on the case, making the case too long.

 

[onespeedbiker]

The 357/30 starts as a 357 mag case, while the 300 Max Cat does start as a 357 Maximum case. The OP description of the case makes little sense. "I don't have a clue about this 357 case. I was sizing and de-priming it and noticed a ring just below the mouth. I used a micrometer on the mouth and it is .356. It also is exactly at the length of a 38 special." So the reduced mouth measures the same as a 357/38 special (.356), meaning the case itself would have to larger in diameter if was reduced to 356 (the size of a 357/38sp bullet). Perhaps he means the OD (not ID) of the reduced part of the case measures .356. Further he indicates the parent case is a 357, but then he says it measures the same as a 38 sp?? My guess is he has a 357 mag case with the none tapered portion is the same as a 38 sp. Further, if the OD of the reduced portion is .356, that means the case would pass through the ridge in the cylinder that is supposed to stop the loading of a 357 mag round.