I should have written more clearly. Once the barrel was clean, I checked for erosion at the breech. The rounding of the rifling was noted in the chamber area, after >2,000 shots. The erosion seemed to have no effect on accuracy (from a rest, 10 shots into about 6" at 100 yards, both before and after the test).
In the past, I've gone as many as 500 shots without cleaning in a rifle with a Douglas barrel, using roughly the same loading regimen. Aside from not showing similar erosion of the rifling at the breech, there were no other differences noted in fouling build-up, corrosion, etc
In both cases, the loads used were such that they were easily loaded without the use of a short starter: the patched ball was pressed into the muzzle with the handle of my bag knife, then seated with the ramrod. These are not "thumb-started" loads, but the only "thumb-started" loads I've ever seen involved the use of Minie balls.
My point is this: it is possible to develop reasonably accurate loads that permit shooting all day without cleaning, that do not require the use of a short starter.
That said, it seems that if early accounts of the use of a short starter are sought, early match accounts might be the place to look. As Daryl pointed out, some of the early accounts of rifle accuracy in matches suggests that a larger patch/ball combination may have been used.