Having recently purchased a 1866 Trapper, I was disappointed to find that the trigger pull was about 9 pounds.
I found that a Colt style Trigger/Bolt spring can be used to replace the factory trigger spring on the 1866. You need to remove the short leg on the Trigger/Bolt spring and remove enough from the long leg to make the overall length about 1 inch. Using the factory screw, install it the same as the factory trigger spring. Then you can back off the mainspring screw about a half turn. This results in a trigger pull of about 3 1/2 pounds with no other changes.
GeezerD