that have external hammers is fairly simple. The H and R type shotguns, which have rifle barrels, usually have the same horrendously strong mainpring that the shotgun has, for no good reason. Shotgun primers are VERY hard to detonate. So you can either replace the mainpring in such single shots, with something "weaker", or shorten it by many coils, and space it with a pc of tubing. The horrendously heavy hammer also needs to be skeltonized, bobbed, etc,to speed up the lock-time.
Both types typically have a lot of overtravel in the trigger, after the sear is released. Such trigger movement is very detrimental to any shooting that isn't bench rest or very coarse combat type stuff. Find a way to either fit a setscrew,or else heat and rebend the trigger to make it contact the frame earlier in the trigger's travel.
If the hammer notch is an obtuse angle (greater than 90 degrees) it will tend to not stay properly cocked, especially with a reduced amount of friction. Friction comes from two sources, the surfaces of the hammer notch and the sear tip, and from the tension of the sear spring.
If the full cock notch angle is acute, ie, less than 90 degrees, the sear has to "cam back" the hammer in order for the sear to scrape its way out of the full cock notch. A headband manifyier and light is often necessary to see these things.
The surfaces of the hammer tip and the full cock notch have to be polished, and "square" to each other. Often times, the pin holes thru them and thru the frame are not at the nice 90degee angle that we want, either. DyeKem layout blue is usually necessary, to show you where the surfaces are engaging.
Often times, such parts are casehardened, and once you cut thru that hardened "skin' on the part's surface, it will wear very quickly. So a torch and Kasenite surface-hardening powder is needed, to put back that "skin" that you cut thru, as part of the trigger job.
it's always best, doing such "experimental" work, to get replacement parts BEFORE you start cutting on them. Remove and set aside (in a baggie, to prevent loss) the original, (proven functional, at least) parts, and cut on the new ones. The new ones sometimes fortuitiously provide a much nicer trigger pull, too. Sometimes the wait on such parts can run into months, and that looks bad to the customer, ya know. Once you have everything ready, a trigger job runs anywhere from .5 hour to 2 hours, at $50 an hour, price it accordingly. A man who CAN do good, stable trigger jobs is easily worth that much, so GET it.