Smokey, my man, where do you come up with this stuff--the internet? Or are you just making it up?
What's the basis of "Hawkens [sic] rifles were meant to be mostly carried on horseback, carried by the horse."
Early trade up the Missouri river was conducted with the use of keel boats, not horses. The hunters that provided the meat for the boat crew occasionally used horses, but primarily hunted on foot. True, Ashley's men, the Rocky Mountain Fur Company men, and the American Fur Company men preferred horses when crossing the plains when they could get them and keep them, but the Indians regularly stole their horses and left them afoot.
The horse wasn't much good during trapping season in the mountains. The snow was often drifted and too deep for a horse to get through. Besides, there was so little feed for the horses that they barely were able to stay alive during the winter. The trappers often had to strip bark off of trees to feed their horses. The mountain men ran their trap lines primarily on foot.
William Sublette and Black Harris made at least two trips from the mountains all the way to St. Louis on foot. They left the mountains in winter, and the snow was too deep and the horses in too poor a shape to try to ride.
People traveling with caravans and wagon trains typically walked alongside the wagons. This was the case for the Santa Fe trail and the Oregon, California, and Mormon trails. Sam Hawken himself walked from St. Louis to Denver during the 1859 gold rush. He was 67 at the time.
A typical Hawken rifle weighs little more than a Lancaster trade rifle. During the rendezvous period, most Hawken rifles were full-stock and weighed the same as a trade rifle.
They [Hawken rifles] belonged to a group called the “Plains Rifles”.
"Plains Rifle" is a modern term. It was made up by collectors in the middle of the 20th century and established in our lexicon by Charles E. Hanson, Jr. with his book by that title. Sam Hawken called his rifles "mountain rifles" in his 1882 newspaper interview. Sam's son, William S., frequently ran advertisements in the newspaper for "Mountain rifles... made to order" after he took over the shop in 1855.
They were mostlay [sic] percussion or late flint half stock guns with stout barrels and big bores, made to take down the larger western aminals [sic].
I'm not sure what you are referring to as "late flint half stock guns". American made flint half stock guns are pretty rare. The English were making them, but by the time half stocks became popular in America, we were well into the percussion period.
Really they were mostly custom affairs aside from some of the bigger names so you can get all sorts of barrel lengths.
I'm not sure if you are talking about your generic "Plains Rifle" or specifically about Hawken rifles here. Surviving J&S Hawken rifles commonly have barrel lengths around 38", some as long as 42". Fashions changed over time, and generally barrels were shorter after Jacob's death. They did make some custom rifles, but records show they made rifles for large orders from AFC and from John C. Fremont that weren't necessarily custom rifles.
The Pedersoli Hawken is based off the J&S St. Louis Hawkens and those were all bespoke rifles.
The Pedersoli Hawken more closely resembles a S. Hawken rifle. It was copied from the Uberti Hawken that was copied from the Cherry Corners/Ithaca Hawken which was a generic S. Hawken rifle.