Now I clean after every range outing so that might be a factor but I have NEVER had an issue with .38 in a .357 causing buildup that eventually causes issue.
I clean after every outing as well. But the phenomenon is real. It mostly occurs with lead bullets. I believe the "crud" is mostly wax lube. From my experience, it doesn't take a lot of rounds either. Starting with a clean gun (always - I have NEVER taken a dirty gun to the range) I experienced this at the range many times. You have to push the 357 cases into the crud to get them to headspace. Everyone has different experiences. From mine, I get what the OP is saying.
Butzbach, couple questions: 1) What gun is going to shoot these? 2) WHY do you want to download to 38+P?
Edit: forgive my short term memory issues (seriously). You did say this:
My edc is .38 +P and these would be my weekly practice rounds.
For me, and for our OP's reason, I chose some time back to not bother shooting 38 cased ammo in my 357 guns. If I want to shoot something softer, I shoot my 38 gun (S&W 67, 4" bbl).
That said, I do "de-tune" a fair share of my 357 ammo. I don't know if they reach down to "38+P" behavior; or, how that is actually defined. But I do have some lighter 357 stuff.
One of my favorite is a 158gn plated SWC (Xtreme bullet co) with 7.0 grains of Unique. Yields 994 f/s through a 3" bbl (M686); and 1058 f/s through a 4"bbl (also a M686). I suspect however, this is a tad above 38+P in terms of both pressure and performance. This is a good practice level round for me. Maybe you too.
If you want even less performance, I would move to something even faster than Unique. From my tests, this 7.0gn charge weight was about as low as I can go and still get a clean burn from Unique. But once you start moving to even faster propellants in the cavernous 357 case, you start getting into very low fill rates, resulting in inconsistent burns. It's been my experience that loading 357 mag with heavy bullets and fast powders can be a little nervy. Pressures can go up quickly with small increases of charge weight. Even after 36 years of loading experience, this is territory I prefer not to tread.
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