Interesting. The French did some experimenting with straight rifling in 1892
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA288 It was believed that long term use would "burnish" the bores and actually improve patterns over time; no mention of worn grooves
30,000 rounds with improved full choke pattern
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA227W.W. Greener in 1907 edition of
The Gun; a gun used by "Schultze" for many years in which the pattern % improved
https://books.google.com/books?id=3HMCAAAAYAAJ&pg=PA353 
