It takes a certain amount of strike force by the hammer to set off a given brand of primer. The hardest primer is generally considered to be CCI, so that would be the benchmark to set off anything that comes along. Federal is the softest, or easiest to set off. Everything else is somewhere in between.
Due to the variations listed above, and many more, you can't tell what the hammer is doing by pulling the trigger. You can isolate just the hammer and mainspring by hooking the trigger pull gage under the hammer, carefully letting the hook of the gage down on the frame, and see what the weight is just as the hook lifts off the frame. That gives you a repeatable way to measure hammer fall force independent of all the other lockworks.
If you want the lightest trigger pull that's easily obtainable, back out the strainscrew until you get misfires, then tighten it back up 1/8 turn at a time until you are getting 100% fires. Then another 1/8 turn to cover that 1 in 100. Measure the hammer tension as described above and record the number of ounces. Shorten the strain screw until it duplicates that number when the screw is tightened all the way down.
Next, get an assortment of rebound springs from 11 to 15 lb. Try them out starting with 11 and working up until you get the lightest one that returns the trigger the way you want it to.
This will tune the springs to your ammo. It does not take into account any problems the gun may have that need to be corrected, or stoning/polishing. Those items are extra. Many guns don't need anything additional, some do.
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