3D Spotlight has posted their No One Lives Forever Tweak guide.

Shell casings. Set this to On to enable the display of shell casings when weapons are fired. This can reduce performance, particularly in heavy battles where many guns may be firing. For optimal visual quality (More realistic) you should set this to On & for optimal performance Off. Muzzle flash lighting. When set to On this enables the use of dynamic lighting when weapons are fired. This setting has a dual effect in that setting it On looks good & can give away an enemy's position if they are firing from afar. Those with slow CPU's should set this to Off for best performance.

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