I'm running a Wolff 16501 XP Carbine spring with a PSA heavy 9mm buffer in a PSA 16 inch 9mm upper assembly and bolt.
With the original carbine spring, the recoil impulse was far too pronounced for the caliber, and I was getting a little gas in the face and a lot of fouling in the action.
Just changing out the original buffer spring for the Wolff XP carbine spring instantly cured all issues.
It actually kept the gun running a good deal cleaner, I assume due to the stronger spring keeping the action closed somewhat longer resulting in more complete combustion with less crap getting blown back into the action.
The rifle has been utterly reliable with every 115 grain load I've tried from IMI +P+ subgun ammo to Wolf steel case to IMI standard pressure and HPs.
These blowback guns run, and especially feel better with a heavy spring and buffer combo. IMO it's a must have.
FWIW!