The rocky mountain trappers had horses. Their "possibles bag" rode on the horse. Their bullet bags or shooting bags were worn over the left shoulder and under their right arm. They never carried "haversacks" those were military equipment.
Basically, the bag worn by those trappers only contained items necessary to operate the gun. Descriptions from the time say they attached gun worms and stiching awls to the strap behind the back.
These days there is much confusion about possible bags. I've tried to sort it out but there isn't a lot of information to be found and the names of those bags, what folks call them, can be confusing. leather bags wore out long before the guns and the few remaining examples are from much latter time periods. These leather bags and powder horns were produced commercially and trappers did not go into the mountains to make equipment. They were there to trap furs. So they started out with bags made by harness makers. They would only make their own if one was lost.
The problem with "possibles bags" I see at rendezvous are two fold. The first is a result of the "mountain man run" competitions and those guys who compete in timed events have more stuff attached to the outside of the bag and hanging from straps than anything that would be practical in the real world.
The second problem stems from the notion that a "possibles bag" would contain every item that a mountain man might need to survive. That sure slows down the loading process when a pilgrim has to dig through all that "stuff" to find what he needs to load his rifle during a shooting match or a hunt.
I don't believe we will ever find enough information to say definitely what was or was not carried in the bag that accompanied the gun. But it seems evident to me that the most practical method is to include only the items necessary to operate and maintain that gun. Long ago I gave up the idea of having only one bag. It just isn't practical to change out the entire contents of the bag when changing guns. Now I have a bag for each gun and I design that bag to best organize those items required for that gun. My 40 caliber Lancaster is served well with a small belt bag and my 62 caliber smoothbore needs a large bag to hold all the items needed to load round ball or shot.
This may not be the most historically correct method, I doubt we will ever know what that was. However it is the most practical method for hunting or shooting a rendezvous event that I have seen.
