All my books are still in storage, so I have to go by memory as best I can. A couple points:
1) A distinction must be made between patched and unpatched ball in 18th century accounts. Mallets and short starters WERE used for starting oversized, unpatched balls - IIRC, there is a French military source from early in the 18th century that describes the loading sequence for a rifled carbine using an oversized ball started with a ramrod and an iron pin. This was later plagiarized by an English source. It may also be the source for Robins' treatise that Seth just posted.
2) The Baker rifles and their associated mallets get brought up very single time, but it is not certain that the mallets were used for routine loading. I believe that DeWitt Bailey notes that they were not issued on an individual basis but only to every second man, which suggests that they were intended to keep badly fouled rifles shooting in emergencies rather than for routine loading. Of course, what the quartermaster intends and how they were actually used may have been quite different...
3) The earliest use of a mallet for starting a patched ball that I am aware of comes not from Britain nor America but from Austria, with the 1796 model military Jaeger with its odd detached ramrod:
4) Despite numerous assertions to the contrary, "they" did write down how to load a rifle: I am aware of at least two accounts of loading patched balls in a civilian context from the relevant period, one from mid-18th century Germany which discusses how to hold the ramrod to avoid breaking it when starting the ball (contained in Wolf's Steinschloss Jaegerbuschen) and Audubon's account of Kentuckian loading procedure around 1810, in which the the patch is cut at the muzzle and the ball started with the knife handle. In addition, quite a number of mundane details about daily life are mentioned in passing in the Draper manuscripts, Shane interviews, Isaac Weld's description how these strange (to him) weapons were used in the 1790s, probate inventories, etc. There is really quite a lot of evidence for how firearms were used along the frontier, including debreeching barrels to unload a bad charge, wiping between shots when squirrel hunting, de-hexing the gun if you happen to shoot at a wolf and miss (Amerindian belief, that one) - you just have to look for it.
5) Modern barrels aren't very good representations of 18th century barrels. Original barrels, by virtue of the manufacturing process, would necessarily feature a slight degree of choking due to wear on the boring and rifling blades and compression of the materials used to hold them against the metal, and we know that different patterns of choking and funneling were deliberately introduced in fowling pieces. Baird's figures for the Hawken rifle he examined (posted by Dan Phariss above) suggests that rifle barrels may have been treated the same way at times - to the best of my knowledge no one has ever researched the question. Ergo, we should probably be extremely cautious about extrapolating too freely from our own experiences with replicas - what is necessary for good accuracy with a modern barrel may not be necessary, or even desirable, with originals.
I think that it is pretty clear that while mallets and short starters were around throughout the 18th century, the use of them for patched balls was a European innovation of the last part of the 18th century and didn't spread to the US until the 19th century. There seems to be a correlation between the use of a mallet and military use, perhaps due to the need to get mass-produced balls down fouled rifle barrels in the field - I suspect that the fact that this innovation occurs alongside the introduction of much larger conscription-based armies and new operational concepts blurring the distinction between the strategic and tactical spheres is not a coincidence (that is, the shift between 18th century "kabinettskreig" and Napoleonic-era warfare). As an aside, based on this I would suggest that the idea that looser-fitting balls were used along the frontier due to the frequency of armed conflict is probably wrong, which in turn raises some interesting questions about how firearms were actually used tactically along the frontier and how both Whites and Indians viewed warfare culturally, but that is kind of a big topic in itself.