Snap, you've received some good recommendations on referenced books that I also endorse, but there are some other books you might consider to fully round out your understanding of the types of guns used in the early years of the "Rocky Mountain Fur Trade".
The single best resource is The Encyclopedia Of Trade Goods Vol. 1: Firearms Of The Fur Trade by James A. Hanson. Other good books to add you your library are For Trade And Treaty by Ryan R. Gale and American Military Shoulder Arms Vol. II by George Moller. And of course Charles E. Hanson, Jr.'s book, The Hawken Rifle: Its Place In History, helps to put the Hawken rifle in perspective.
It helps to visualize the Rocky Mountain Fur Trade as an extension of the the fur trade that had existed in North America since the 1600's. Trade guns played a significant role in the full history of the fur trade. The market for trade guns was probably second only to the market for military firearms in the 18th century and early part of the 19th century and influenced the development of firearms.
The early records of the American Fur Company, from 1808 to the early 1820's, were destroyed by fire and records of other early fur companies are scarce. As a result, it helps to look at records of the Hudson Bay Company and British and American government purchases to see what type of firearms were common in the early fur trade. Private fur companies were purchasing firearms from many of the same gun makers as the HBC, the British Office of Indian Trade, and the American Office of Indian Trade and later Bureau of Indian Affairs.
Smooth bore longarms were a staple of the fur trade with the dominate arm being the North West trade gun developed by the HBC. Other smooth bores were common such as the typical English fowler and the English Type G or Carolina trade gun. And we shouldn't overlook French trade guns since they controlled the Mississippi River region for so long.
Some knowledgeable experts suspect that the Type G and other English fowlers played a big role in the development of the American longrifle. They believe the Indians and the Indian traders wanted a rifle that had the capabilities of the Jeager, but the handling characteristics of the Type G trade gun. Pennsylvania and possibly some Southern gun makers were eager to satisfy this market demand.
Records of the British Office of Indian Trade show that the British were exporting English trade rifles to America during the Revolutionary War, but there are indications that English gun makers were already supplying these trade rifles to private traders prior to the War. George Shumway has documented several types of these English trade rifles in a series of articles for the Buckskin Report in the early 1980's and updated the material for the Proceedings Of The 1984 Trade Gun Conference which was published by the Rochester Museum & Science Center.
Surviving documents indicate that the AFC ordered all their NW trade guns and their trade rifles from England well into the early 1820's. The early English trade rifles are copies of Lancaster longrifles. Later versions incorporated some military characteristics to reduce manufacturing costs. The last version of the English trade rifle was copied by J.J. Henry in 1828 and became known as his "Old English" pattern rifle.
The fact that the English chose a copy an early Lancaster rifle during the Revolutionary War for the Indian trade indicates that the Lancaster rifle was already well established as a standard in the trade prior to the War.
In 1875, the US government formed the US Factory System to trade with Indian tribes. Interestingly enough, some of the first rifles issued to Indians through the Factory System were US 1792 contract rifles. These are the model that some believe that Lewis and Clark took on their Corps of Discovery Expedition. Little else is know about the other rifles purchased by the US Office of Indian Trade prior to 1803. Some apparently are similar to the English trade rifles and others were likely "similar to commercial rifles made in Lancaster and York, Pennsylvania" [Moller].
In 1803, the US government contracted with Dickert, DeHuff & Co for trade rifles. Gun makers associated with this company were Jacob Dickert, Henry DeHuff, George Miller, John Bender, Christopher Gumpf, and Peter Gonter. Other purchases were made from gun makers Jonathan Guest, Joseph Henry, John Miles, and Henry Pickel. Many of these same gun makers were supplying rifles to various private fur trading companies, also.
In 1809, Henry Deringer entered into his first contract for rifles with the US Office of Indian Trade for the Factory System. Deringer would soon become the dominate supplier of rifles to the Factory System and later Bureau of Indian Affairs. By 1822 when William Ashley and Andrew Henry advertised for "Enterprising Young Men" to ascend the Missouri River, thousands of Deringer rifles had been distributed to US Trade Factories from the Great Lakes, along the Mississippi and Ohio Rivers, and all along the Southern frontier from Tennessee and Georgia to Arkansas. The majority of these rifles were traded to Indian customers, but a number would have ended up in the hands of non-Indians. These would have been "Border Men"--the very type that answered Ashley and Henry's ad. So it is very likely that Deringer rifles were present. It is also likely that Delaware, Shawnee, and other "removed" Indians that participated in the Rocky Mountain Fur Trade carried some Deringer rifles. An interesting side note, several hundred Deringer rifles intended for the Factories were issued to Andrew Jackson's men during the War of 1812 and were used at the Battle of New Orleans. Some of these rifles may have eventually made their way to the Rocky Mountains, also.
To sum up, smooth bore guns used in the Rocky Mountain Fur Trade from 1825-1840 were predominately North West trade guns with some English and American fowlers and a smattering of French trade guns.
Prior to the Revolutionary War, the early Lancaster longrifle was established as the standard for the Indian trade. As the Pennsylvania rifle continued to evolve in the early 19th century into what we commonly call the Kentucky rifle, a very plain version became the "pattern" for the fur trade. Records from the American Fur Company and other fur companies in the late 1820's to the 1840's referred to this as the American and Lancaster pattern, interchangeably. This would have been the most common rifle in the Rocky Mountain Fur Trade. English trade rifles, both imported and Henry's version would have likely been second. Philadelphia rifles from Deringer and Tryon would have been present in significant numbers. Hawken rifles were relatively scarce in the Rocky Mountains in the late 1820's, but gradually increased in the 1830's and more common in the 1840's. Even though a few Hawken rifles are mentioned in inventories for the rendezvous and Missouri River trading posts, more Hawken rifles probably went west via the Santa Fe Trail to Bent's Fort and Taos. By the end of the beaver trade and during the robe trade, more Hawken rifles were likely present in the southern Rockies and the plains from Fort Laramie to Bent's Fort than the northern Rockies.
