In a modern firearm the trigger and sear are all tied together, making it easier to get a good trigger pull. In a black powder rifle, the trigger is set in wood one place, while the lock is set in a different location. The reason a double set or even a single set trigger works well, is because the set trigger, loads the trigger arm under spring pressure when the set trigger is used. Upon releasing the main trigger, the arm flies up striking the trigger bar. On a single trigger, the geometry must be so as to allow for optimal pressure to press against the pressure of the main spring and the engagement of the sear and full cock notch. Some people know how to achieve the geometry, others don't. The set triggers are simple to set. In a single trigger, reducing the spring pressure will lighten the trigger, but will also slow down the lock time. Reducing the sear engagement, might result in a dangerous situation, and in some cases, ruin the tumbler, all together. As mentioned, repining, and correcting the geometry is the correct way to achieve a better pull. For me, I'll stick with the set triggers. I shoot enough, that it is second nature, and I have never spooked a deer, while setting a trigger.
