Brass isn't as good a guide material. The 660 bronze is a type of "bearing bronze" that has reduced friction properties. I think most brasses are also generally softer than bronze. At any rate, the guide would need to be fitted to your spring (whether stock or aftermarket) for best results. Certainly not necessary (delrin would work just fine also) in a gun of this power level but can't hurt.
Regards,
Warren
