Wow, lots to catch up on on a snowy day along the Applegate Trail ...
I'm drawn to the 1841 Mississippi as a very useful rifle well rooted in the period. Another fascinating rifle is the Wesson boxlock. The actions are available but you'll have to have a smith build the whole rifle. Here is a shot of the butt end of one supposedly carried by Texas Ranger Jack Hayes circa 1849. Note the lollipop tank rear sight:
Years ago, I served as the guest firearms curator for the Southern Oregon Historical Society, which has an extensive collection of original firearms from the 1840s-1890s. There was a tremendous amount of variation among the "Plains rifles" in terms of caliber, barrel length and thickness, conventional lock vs. back-action lock, etc. Unfortunately, owing to financial difficulties the firearms collection is in storage.
Regarding rules, my take is basically "PRAISE BE TO GOD WE AREN'T INTO FASTEST TIMES," from which a whole host of negative issues would be likely to spring. Honestly, back in the day, your shot had to be accurate to tell.
I would also like to see continuing flexibility on the arms required; too many folks get discouraged having to buy two pistols and rifle and a shottie right out of the gate, and if you surveyed every wagon on the Oregon Trail circa 1862, I'll bet you'd find very few that had this arsenal under the hemp canvas ...