Couldn't agree more with Northmn that military arms have to be more robust and able to take more punishment while needing less maintenance, than most civilian arms. That doesn't mean civilian arms can not or were not used effectively in combat, whether 18th century or later, it just means they are usually going to break down faster than rifles designed for military service.
I hope no one takes this to mean I think 18th century civilian rifles were in some way "weak." Those who settled beyond the mountains or Long Hunters had to have a rifle that kept on working as it could be a long time before they saw a gunsmith. Side note: I've often wondered if at least some of them had gunsmiths fit a replacement set of springs and maybe a sear and kept it with them in case their rifles broke down? I wouldn't want to go out on a "long hunt" without some spare parts, but maybe that's because I've spent so many years fixing guns.
However, civilian rifles are never fired as many rounds/as often in civilian use as rifles or muskets are fired in military combat use. It was quite possible to probable that American Longrifles were fired more in two or three campaign seasons than they would have been fired in 10, 20 or maybe even 30 years of civilian use. That means more wear to the locks especially and more break downs. Whether due to style, individual taste, etc. reinforced cocks were not
commonly found on civilian arms nearly as soon as on military arms. Other military lock parts and springs were usually more robust to stand up to more use. I'll grant you there isn't a huge difference in the size of military vs civilian lock parts in rifles, though.
I really don't have a lot of good evidence on how much weaker civilian stocks were at the wrist than military arms and if there was a noteable difference in actual use in the mid to late 18th century through the early part of the 19th century. Because most American riflemen in the Rev War did not have bayonets, we know they sometimes had to resort to using "clubbed rifles" and that can break a stock wrist pretty quick. This doesn't mean a stock wrist is weak, though, because the wrist of a civilian stock was never meant to withstand that kind of force. A Brown Bess or Charleville musket wrist was not that much thicker or more robust and with the heavier buttstocks, they may have broken as often when using clubbled muskets.
Zebulon Pike's 18 enlisted men broke five rifles on the march and bursted three other rifles (I assume they meant burst barrels) during their expedition 1806-07 and the
Author of Guns on the Early Frontiers" believes those were M1803 rifles. What I don't know is what part/s of the rifles they broke. The Lewis and Clark expedition also broke rifles and they carried spare parts and fixed them.
http://books.google.com/books?id=BJRJZZIxrmkC&pg=PA180&lpg=PA182&ots=XsjJr-A7jQ&dq=harper%27s+ferry+model+1803+rifle&ie=ISO-8859-1&output=htmlThere has been much discussion/debate recently on whether Lewis and Clark actually took M1803 rifles with them and some of the information on which rifles were actually carried was not available to the author when he wrote that book. I'm guessing the Zebulon Pike expedition did carry M1803's though, because production was going on in time for them to have been issued those rifles.
The underrib on the M1803 was at first seen to have been stronger than a full stock and seems to have been "the trend setter" for civilian rifles later on out west. Arguably a noteable example of the conservative military minds doing something "fairly new and improved" in a big way before civilian arms. They did away with the underrib on the M1814 and M1817 rifles though and it was not seen again on most U.S. Military Rifles in the ML era through the M1903. Was this mostly because they envisaged usage of the M1814 with a bayonet and we know they intended the M1817 to be used with a bayonet similar to the M1816 musket, though it seems few bayonets were used or even issued for the M1817? Was the underrib just too "new fangled" for many minds? I admit I don't know.