I shot TC guns for years using the "P" stuff, didn't know better. I shot my gun out at the end of the day and gave the barrel the soapy water bucket douche job as soon as I got home, I never had any rusting or pitting.
I went to the dark side for years with one of those scoped new fangled thingies some call a M/L. Common sense prevailed and I came back to my roots, I found I could buy BP online and never considered using the "P" stuff again.