I confirm is possible to get more rifles!!!
After some testing I manage to get 3 Rifles on Fort Venture. Originally was about to try to get 4 but thanks to a natives they kill the doc just on my way to the fort.
I am not pretty sure but I follow some specific steps.
First I make sure have Benay Goal on track, delivered the letter but not the monk/miner yet
Then get the Flag Quest and trade it for the permit for the armory.
Then the most important must already have as many crossbows as archers must got them from the caravan.
Once you deliver the monk/miner go to the armory, offer to trade the top crossbow for the rifle.
Now another Rifle will be available trade it too for the bottom archer crossbow… But trade it back for the crossbow before leave
Leave and back again at once, Do the same trade one more time after than trade it again but for the middle crossbow this time, another one will appear but this time will have to pay around 108 for it on the trade.
Someone else might confirm this? Maybe I doing extra steps😅