Here is a little info from a freind of mine, Charlie Starks, about full and half stocks relative to traditional muzzleloader history. He is on the board of trustees for the TraditionalMuzzleloader Association http://www.traditionalmuzzleloadingassociation.org/
He knows his history and is also a great part time gun builder & gunsmith.

"well both have been around along time so to narrow it down some we
must ask where , what country we are talking about . german and dutch
makers made some very fine half stock rifles and fowlers as did the
french i believe.

here in the Americas thought Its my understanding that the half
stock or plain rifles didn't real become to prominent until about
the time of European involvement onto the plains .
Before them full stock was the order of the day . Sure you will
find original trade guns that are half stock however I believe this
was from damage being done to the forstock and thus the stock
shortened .
With everything else though even all full stock rifles are not the
same . Commonly English rifles as well as some American made ones
can be found with heavy fronts while those with French and German
influence are normally very light on wood in the front . This also
changes with grade within the same rifle line so its not a stead
fast rule

There still is a lot of discussion of why the difference in weight
and length to the shorter and heavier weapons of the plains . Many
think this was due to the use of the horse . While others lean to
the greater knowledge of ballistics and how weapons work thus
making the transition a natural evolution of the weapon
What we do know when it comes to the plains style rifles that the
most popular know of them , the hawken
While probably sought after ,was most likely not very prevalent
during the fur trade . There has been a lot of information put out
about the hawken bros here recently and the common consensus seems
to be that the hawken rifle " or what we know as the hawken"didn't
come to prevalence until about 1842-43 and even then the bros were
only putting out close to 100 weapons a year while other makers like
Larson and Wilkins were putting out close to 500.

One thing is we are living in interesting times right now , lots of
people digging into history now so a lot of new info is coming to
light . What was common knowledge yesterday is not being debunked
today"