So I am going to order a new recoil spring for my single stack 4013, as recommended by many in this forum. I have been looking on the Wolff site, and have also read members here saying they use the next step up from stock, 2lbs. extra rating on the spring. My question... what is the benefit over using a stock/OEM rated spring, and what are the effects? Thanks.

I can see no reason to install any spring other than the factory-designated weight unless you are going to be shooting ammo other than what the gun was primarily designed to fire.

Weaker springs for light loads.
Heavier springs for heavy loads.

Its not a complicated analysis.

If you switch springs, thoroughly test the spring in your gun for reliability.

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WWSSD?
What would Skeeter do?

Ok, thanks. Is it safe to assume a 4013 is designed to shoot 165gr and 180gr, like the typical ones you find at Walmart?

He's not really referring to bullet weight but other than full power standard loads-like you ususally find at big box stores. Some calibers have target loads with reduced velocities/recoil to make it easier to shoot groups. The +P loads are heavier than standard.

Thank you as well.

I changed the spring in my 10MM to a heavier unit to keep from flinging brass into the next zip code. I shoot and reload and the somewhat worn stock spring tossed the brass around 10 feet away.

That was a PITA to police the brass at the range. The heavier spring reduces the distance the brass flies.

I'm shooting pretty standard to mid-heavy loads. It hasn't diminished the reliability of the pistol at all. The lightest rounds I shoot are 155 gr at around 1150fps.