I put a lighter Wolf rebound spring in my pre lock 317. The trigger was very heavy and it helped somewhat.

This got me thinking about the purpose of the rebound spring. I realize it has to reset the trigger. And it serves as a "safety" in some sense. That is, a heavier double action pull would be safer.

Are there any other functions?

Why does S&W use such a heavy one?

If you shoot fast as in Bang, Bang, Bang, Bang, Bang, Bang -- the heavy spring helps assure positive reset. You'd be surprised at how hard it is to shoot really fast unless you have that spring pushing that trigger back out all the way -- a weak spring is slower and does not fight the tendency of the shooter to start pulling before reset.

All that is to say unless you want to shoot at a rate of about 6 shots in 2 seconds, it probably won't make that big a difference.

The rebound is the main safety in the S&W design.

When the trigger is released the rebound "box" slides forward.
A lug on the rebound engages a lug on the bottom of the hammer and that forces the hammer back away from the cartridge. This is known as a rebounding hammer, and thus the rebound slide and spring.

Once the hammer is rebounded, the lugs on the rebound and the hammer interlock to prevent the hammer from moving forward again until the trigger is pulled, pushing the rebound backward and unlocking the hammer again to move forward.

Since the rebound IS the real safety the spring must be strong enough to force the hammer back no matter how dirty the action is and strong enough to prevent the hammer from forcing the rebound out of the way if the hammer is struck a hard blow.

The rebound spring also powers trigger reset. The spring must also be strong enough to force the trigger forward quickly and positively.
If the spring is too light, you can have problems with "short stroking" the trigger, which is not allowing the trigger to move far enough forward to reset the action.

So, the rebound spring serves the purposes of rebounding the hammer, locking the hammer so it can't move forward, and resetting the trigger.
A light rebound spring can make the trigger action lighter and easier to operate, but it also slows down the speed of the action, which is why some speed shooters install stronger rebound springs, not lighter.

These days we understand that the lighter the trigger pull is does not mean a better trigger. What you want is a SMOOTH trigger action, which is not a function of lighter springs.
A smart gun owner stays with the stock rebound spring in a real world defense gun.
Light springs are okay on Match guns and range toys where your life isn't at stake.
A smart shooter builds up his trigger finger strength with practice.

Thanks wheel. You da man. I'll probably be putting that original spring back in.

You're welcome.