The 1816 muskets were manufactured (about 700,000 of them) from their inaugural date till 1844 at the Springfield and Harper's Ferry arsenals. It was superseded by the 1842 (percussion) musket and m1841 rifle. Conversion of the 1816s to percussion occurred through the 1840s to 1860s. There were different types such as the Cone in Barrel (1840s), French Drum (1840s) and Remington "Maynard" (1850s) conversions.
Although a lot of US Army weaponry was made available to the growing number of emigrants headed West (eg. "Mississippi" Rifles, Flint & percussion muskets, horse pistols, Hall rifles/carbines, etc. with accouterments as well) I feel that we should not skimp on representing the vast number of privately made, rifles, fowlers, muskets, combination guns etc, that were documented to be out west either.
In regards to Militias, the threat of secessionists did not really become a big scary deal until after the presidential election of 1860. Militias were used to supplement the region's military presence in the event of war, social disorder or natural disaster. There was no International Red Cross yet so it was up to the Militia to restore order and conduct rescue operations in times of need.
Word of mouth accounts such as family history's tend to be vague in their accuracy at best so unless you have first hand provenance of what a family member claims about his old great grand pap's gun, it is little more than family legend. (Ex. My grandpa showed me his 1873 Springfield Trapdoor rifle claiming that it was from the Civil War (1861-65).) Although I grandpa was clearly wrong, the best I can say about family legends is that the story may be true and it may not be (quien sabe?).
The best references for provenance of firearms types in any location would be archaeological digs, first-person accounts, newspaper advertisements & official records/returns. When it supports the family claims behind a historic gun, it enriches the mystique of the piece as well as gives it a higher-than-usual auction price.
I hope this helps, friend. If you are talking about a specific gun you have, I'd love to see pictures.
-Dave