I've wondered myself where the length of some of the longrifles came from. The eadly Dutch settlers of what is now New York had fowlers with barrels of 50 to 55 inches! Makes a 46" rifle seem a carbine by comparison. I wonder how much the military muskets of the day set the length that became fairly standardized. 46" is about as long as a 'normal' height man can load and handle efficiently, while still giving a practical pike with a bayonet attached.
I've had the good fortune to wander through a few forests locally (New York & Pennsylvania) that have never been logged. The trees are about 15 feet apart and there is very little or no undergrowth. Certainly not a problem for even the longest of barrels. But even by 1750 there was a signifigant amount of clearing around settlements, and besides, the mature old, growth forests are pretty devoid of game because there is no cover or browse, so the glades, glens, creeks, hollows, etc, where there is cover and browse is where you hunt and enemies hide. So why weren't the rifles using 24" barrels to make them light and hamdy?
When you're loading a gun with the buttplate resting on the arch of your foot the muzzle should be at about elbow height to give you the optimal leverage when loading (my own ergonomic observation). Hey Presto! That gives a 28" barrel with a 14" length of pull for me. Plains rifle dimensions.
I've read theories that barrel length tended toward longer because the early powders either had to spend three months at sea or be made domestically with less than optimal ingredients (and stored in community 'magazines' of dubious integrity. Perhaps? There is also some truth to the far forward sight being an aid for low light and a help to uncorrected vision abberations.
Then there's the weight issue. Swamping the barrel helps tremendously, but a 1782 rifle believed to have belonged to Daniel Boone still weighed eleven pounds with it's 48-3/4" .44" cal. barrel (1" across the flats at the breech). That's a heck of a thing to drag all around Tennessee and Kentucky. Something had to have dictated that length of barrel? Style? Effectiveness as a club? Status symbol? We won't even go along the Sigmond Freud theories.
I'd love to be able to ask some of the original smiths why they did what they did.