I find this discussion interesting and maybe even a little sad too.
For the pro short starters, there is the absence of documentation but I think more importantly, nothing, at least none that I know of are recorded as relics. The same can be said for rifleman's mallets. No description, no relic at least in the American frontier period.
The anti short starters of course have the above on their side but they to grasp at things just to prove their point. IMHO.
Course tapered muzzles where used quite a bit back in the day minimizing the need of a short starter.
I'm not convinced of this. Maybe I have not run across it yet so I may need to hit the books some more but in my studies of rifle building I have yet to find period mention of coning a barrel.
Decorating the muzzle, Sure, it's there. Coning the first few inches of the bore, Nata, in what I've read. If this was a common procedure it seems like it would be mentioned in the manufacture and "repair" like refreshing the barrel.
So what about these rifles that have what appears to be coned muzzles? Could it simply be wear? Wear from the rammer or gritty patch? Wrought iron is a good bit softer than steel. It is pretty rust resistant, could it be that barrels that needed refreshing were from wear, not corrosion?
I have read in print that the patched ball was an American innovation of the rifled barreled gun. Of course we know that is not the case and the only thing that can be claimed as American is the patch box but now that is in doubt.
Should we discount those statements by earlier scholars about the American innovation of the patched ball?
No I don't think so. The Americans may not have invented the patched ball but they used the concept to make the rifle, which in Europe was a slow loading toy of the gentry requiring the ball to be hammered down with a mallet, into fast loading viable combat weapon.
How was this accomplished, by the patch and ball. If the ball was small enough not to require hammering it down the bore, why not take it just a step further by making it small enough that no tool is required to load it, even the knife?
In period accounts it is remarked how small the ball fired from a rifle was. It is a common assumption that this means bore size compared to the large bore of the musket. Could these statements of ball size mean that the ball was a good deal smaller than the the size of the bore and not simply a comparison between the musket and rifle's bore size? Meaning that the greased patch of linen or other documented material like buckskin filled the gap between the small ball and bore.
If this has truth in it, that could mean that no coning was required, no short starters, no mallets, and no knife butts were needed to assist loading.
So the answer could simply be this, the reason no short starters are documented, is they simply were not needed in the unique way Americans loaded their rifles.
The ultra tight loads, thin patches and tight balls could be an innovation from late 19th Century match shooting like the false muzzle.