The Musket is

Jezail Musket

a weapon that can be sold for at least $60,000 to in-game merchants. But due to the nature of the gang, they do not realise the treasure they were looking in the neighbour (the veteran's) of Lis house would be this thing, taken from the Afghan wars. In the missions they either throw this weapon away or use it as a meelee weapon due to lack of knowledge on how to reload muskets after the shot.