You probably already know this, but if you shoot a bunch of .38 out of a .357, the .38 can leave a ring of crud in the cylinder charge holes that can make it difficult to chamber or extract .357 rounds. The .357 cases expand and lock into the crud ring when fired, making extraction difficult. The cylinder charge holes need to be pretty damn clean. I use a .40 or .410 bore brush in my .357s when shooting .38 to make sure that I get all the crud out.
Another good option to get all the crud out after shooting .38 if you are a reloader is to get some .357 cases. Use your reloading press to slightly bell the mouth of the case until it fits into the cylinder charge holes pretty tightly. Then shove the .357 cases into the charge holes. The belled case mouths will scrape the crud out of the charge holes very well. If the cases fit tightly enough, use a cleaning rod to push them back out of the charge hole. Then use a little Hoppes (or your cleaner of choice) on a .40 or .410 brush and the charge holes are clean as a whistle. If you're not a reloader, see if you have a friend that is and can bell you some .357 brass. Very easy and quick.
Bub75