Goldenage,
By limiting to flintlocks, it is just a simple way to keep firearms authenic to the peroid. People like stuff simple, brief and easy to understand and by making rules like this one it is as clear cut as it can be.
The bad side is that by doing so, they limit the education and fun for not only the general public but the historical community. There are many, many guns that fit into our time frame that are not flint (you have conversions, cap lock....). But many do not want to take the time to find out what is apporiate and learn about this. Plus it keeps out modern guns that are nice guns to have and shoot, but do not have a historical heritage (modern guns that do not fit into classical styles).
Nothing too important about the 1840 cut off date except what others have written. People were still going west; adventures were still goiing on, journals kept and alot of exciting day still happened well after the Jan. 1st, 1841 date.
mike.