I never slugged a barrel before, and slugged the LCR 9mm today. There was a bit of a learning curve and I'll do it again, but I think I found the culprit of the leading from shooting the 380 95gr LRN bullets, and that would be oversized cylinder throats, NOT oversized groove diameter.
From what I could tell the groove diameter is somewhere just shy of .355", closer to .354". That surprised me, but that's about where it should be.
The slug would just about swish through the cylinder throats, and a .358" 38/357 LSWC bullet would pass through with light pressure, and the cylinder wasn't super clean.
Maybe the lack of the bullet sealing in the cylinder allows some lead to be blown around the bullet and into the barrel. And/or maybe enough lead is also removed to prevent the bullet from sealing in the barrel.
I'm going to try some MO Bullet Co's 124gr LRN with the "Hi-Tek" coating, and see if they eliminate the leading.