Could single base powders such as PB and/or 7625 cause rust in the chambers?

Today we noticed a coat of rust in a friend's Fox XE chambers, he reloads with PB. Another friend has reloaded with both PB and 7625 and he, too; has experienced rust in the chambers of one of his Parkers and speculates that these single base powders could be the culprit. Both of these fellas are meticulous with their gun cleaning regimen and do not store their guns in a damp environment.

What are your thoughts on this?

Wild Skies
Since 1951