I am replying for my husband who is a muzzleloading riflesmith:
 
Trade guns were regularly shortened, both by Europeans and Native
Americans.  In their original length, they were awkward to use on
horseback.  They may have been shortened, even if undamaged, to suit
a different purpose.
 
Toosla (sp.) arsenal:  French arsenal regularly patronized by
Russians supplied such guns, and, therefore, may have entered America
from the Northwest.
 
In the catalog for the show, "Crossroads of the Continent," a Russian
musket shortened by Eskimos is shown.
 
If the section of the barrel has been removed by a chisel, it most
likely was by a blacksmith; either a Native American or a smith
utilized by local tribes.
 
If turned on a lathe, a gunsmith likely did the work.
 
Anything French may have appeared in any NA context early on, as well
as have been readily available to the American Colonists.
 
Enlish soldiers most likely carried Long-pattern Bess--a general
military issue.