A lighter weight rebound spring will lower trigger pull force, but at the cost of a less forceful trigger return/reset.

For gun carried for self defense I would not reduce the trigger rebound spring weight. If anything I'd add a few pounds (not really)!

The reason is thus: About the only "malfunction" you have to worry about with a revolver (non parts breakage) is short-stroking the trigger during rapid DA shooting. Granted, the M351C is a small gun, but under the stres of "fight or flight" where the body dumps large amounts of adrenaline into the blood, fine motor coordination decreases as gross muscle strength is enhanced. This is why the things you can do when calm become a lot more challenging when you're heart is suddenly racing 120 bpm and you're scared, and you're hand start shaking.

With a DA revolver there are two things that can go wrong with the cylinder locked in place. Your release is just short of full reset - reset meaning the "hand" has reengaged the ratchet, but the DA sear has not reset. At this point, a pull on the trigger will cause the cylinder to happily rotate, while the hammer only "bobbles" in place without being cocked and of course the gun does not fire. Not only can this be done once under "stress fire" it can be done repeatedly before the person realizes the gun is no longer speaking.

The second thing is far worse, potentially catastrophic. Just "forward" of the failed reset point mentioned above, the DA sear achieves a mechanical interface with the trigger "nose" (part that drives the DA sear up during hammer cock), and if at the precise location the trigger is pressed - again under the much stronger force of a full adrenaline stress fire situation, the two parts will meet "dead on" and the trigger will "jam" - no movement, no cylinder rotation, no hammer cock, the gun is fully LOCKED UP as long as the finger maintains pressure. When we're playing on the range and this happens, we just relax pressure, the trigger resets and we're gtg. But in a combat situation the human finger can snatch that trigger with such force so as to break the gun internally, which means no more bullets being launched. With an alloy frame revolver such as the 351C the mostly likely break will be the hammer pin being snapped out of position, or the small "nose" of the trigger being grossly deformed. At this point, the gun is out of action.

Anyone can create these malfunctions by pulling the trigger, and easing the trigger forward to just shy of full reset, then pull again. Once you find the "spot" you can literally rotate the cylinder without lifting the hammer or place the internal parts in a bind - at will. Granted this is an "area" in the trigger's return travel that it generally passes through quickly, BUT, both issues are "located just inside" full reset!

Okay, so how does this matter to lowering spring return rate? Because now the trigger has less spring-force pushing it back to full reset, which means is WILL be more sluggish, slower, and "increase" the probability of a trigger pull while the internal parts are still within that malfunction zone. One can presume that with smaller revolvers the "odds" of short-stroking a DAO trigger are less than with larger revos with longer "pull," but when the trigger return force has been reduced, the odds of this go up.

All that long-winded explanation was to say, don't reduce your trigger's return force on any gun you MIGHT carry for SD. And I would think the 351C is such a revolver. This little "quirk" of double-action revolvers is why I often remind people that the ONLY guaranteed shot you have from a DA revolver that you train to shoot DAO, is the first one. All after that are subject to the engineering limits of the gun, as negatively impacted by the shooter! Since the cylinder rotates off the trigger, the ultimate reliability of the DA revolver is tied to it.

Just to point out the difference, with a single-action revolver, the cylinder is rotated when the hammer is cocked and the trigger (in it's simplest form) only releases the hammer - you don't even need one. For this reason, when you start a gunfight with a single action revolver you have ALL SIX for sure! There isn't any hidden internal mechanical interface that can render your gun out of action. Based on this, the best carry revolver is of the Schofield design - top-break, auto ejection, fast reloads, and SAO...

So consider carefully any changes that reduce the speed and force of trigger rest in any DA revolver.