It may be that as the frontier advanced, locally produced iron made the use of brass too expensive further west (Kentucky, Tennessee, SW Virginia) by gun and barrel makers as the gun buyers on the frontier were a different folk than those in the settled east for the most part, with times of peace and stability having a positive impact of course.
Caliber-wise, my personal belief is that once sure man-killing larger calibers were no longer needed as a general rule, bore sizes tended to reduce. And perhaps this and the iron production cycled following declared and undeclared frontier warfare and national wars up to , say, 1820, showing the changes we note in stocks, barrels, calibers, ornamentation, materials. Lon.