DEE,
I borrowed this from the Marlin Owners forums. It's a reply that I wrote over there about my experience with this. I just copied, and pasted it here because I type with one finger, and this is easier than writing it all over again.
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Well, you'll find probably 100 guys that'll tell you that the only problem is a hard fouling build up at the front of the chamber that'll cause 357's not to chamber untill it's scrubbed out. This is true, but "Definitely" not the only problem that can occur from shooting 38's in a 357 chamber.
38's, especially when loaded at lower pressures can "burn" a nice ring into the upper wall of your 357 chamber. I think it's a combination of not enough pressure to seal off the gasses from being forced back past the case mouths, and a powder with a high flame temperature. I'm only speculating on the cause, although from the evidence I've observed in two seperate rifles, I feel sure this is it.
The first rifle was my own(the last one I owned). I had fired pretty much nothing but full house 38 specials through it for at least a couple years when I got the notion that if I found just the right sub-sonic HP load, it'd be perfect for night time predator hunting in populated areas. After several weeks of experimenting I had a very nice cut in the top third of my chamber that matched up perfectly with the case length of a 38 special. This was no "fouling build up", but a "cut" that was a good .003" deep.
It's a shame that I didn't actually notice it at that point. One of the last brainstorms I had while working on that perfect sub-sonic load was to trim my 38 cases back to 1 inch, and try a slow burning magnum powder just to see how slow I could really get a .357" bullet to leave the barrel while maintaining a consistent velocity, and some form of accuracy. I shot exactly 50 rounds of these 1 inch cases packed full of IMR-4227. I can't remember the exact bullet, or primer I used now.
After abandoning this quest for the perfect quiet load, I decided to switch back to full house 357 magnums for some reason. I was short on brass, and when I'd fired all I had, I resized them, and noticed a crack in one just about where the base of a seated bullet would be. When I checked the rest, I found that every single one had two shiny rings about a third of the way around them, and about .01" apart. Of course when I checked my chanmber I was shocked to find the reason for it.
The amazing thing was that the shorter cases did more damage in 50 rounds than all the other 38 cases combined. The ring at the end of the 1 inch cases was at least .005" deep or more.
I showed it to quite a few people till a friend of mine told me I was crazy even after seeing it for his self. He bet me that I couldn't duplicate it in his rifle. I loaded up exactly the same load in the 1 inch cases, and proceded to burn exactly the same ring in his barrel.
Sorry so long, but I felt like you, and anybody else that doubts this should at least know what I've found out about the matter.
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Squealing rabbit, sneaking predator, 357 rifle, and frozen fingers. It just don't get no better than this!