Any negative to using the 357 cases for lower powered 38 sp type loads other than the powder issue of using a little more b/c of the extra case space?

Staying with all 357 brass seems to make more sense for me ... Seems to be a small price to pay (little extra powder) for not having to deal with 2 sizes of brass.

Anybody hazard a guess how many 357 cases I can obtain in swap for 500 once-fired 38 sp, all tumbled, some de-primed?

You got the idea - extra case volume = extra powder to hit the same mark. No other side effects, other than shooter's bliss.

To the right trader, you could get one for one, to the wrong jackass, you'd only get one for two.

What headstamp 38's are you trading? And what 357's are you looking for?

The 500 38's I have are all mixed brands, tumbled, and some are de-primed. I haven't even busted open the bag I purchased 2 years ago when there was no 357 to be had. In desperation I made a knee jerk purchase.

I'm not looking for any specific used cases as they will be used for range practice. If I could get a bag of 357's for my 38's I'd organize them by maker and then stuff them with hard cast.

I already have about 350 brand new cases (200 Winchester nickel cases and 150 Remington brass cases) that I'll be loading with some personal defense loads while I work up a favorite.