The only other possibility that hasn't been explored (unless I missed it) is if you shot the 38sp lead loads first, and they were undersized, causing severe leading of the bore.
After that, not much of anything will shoot right.... (ask me how I know
)
Although, that is really bad grouping for 15yds.... I'm not sure if severe leading can cause it to be THAT bad.
If the bore isn't heavily leaded, I would send it back to Ruger... something is definitely wrong.