Ok....I'm not real sure how to respnd..exactly...so:
Spring tuning in general would be modifying the spring for less strength
assuming a given free length. The free length is the spring at rest and not under load. So the correct way to do this on a coil spring, that has the needed free length for the function at hand, is to reduce the diameter of the spring's wires by "turning" or "spinning". To spin a spring, you support the spring on an axis somehow and use friction against an abrasive, like a sanding wheel or belt. This takes some skill to uniformly reduce the diameter of the wound wire over the entire length of the spring. Removing coils shortens the free length and the distance you have to compress the spring for a given strength. Power of springs has to do with the diameter of the wound wire, overall diameter of the spring, free length, and the number of coils.
A pretty good rudimentary explanation of springs, in general, is on Wolff's FAQ page on their website:
www.gunsprings.com. You do not want to weaken the hammer spring in the SKS any more for reliable ignition of your ammo's primers.
I infer by your reference to "6 pounds", etc. that you're likely talking about trigger pull weight. If so, it is a function in the sks of the kind/magnitude of engagement of the hammer and sear, sear spring, and to a small degree the hammer spring.
If the steel is fatigued, the spring in question is toast. The best springs today are chrome silicon.