Melonite is just a trade name for a finishing process that involves salt bath ferritic nitrocarburizing, the other trade name for it is Tenifer (like used in Glocks and other weapons). The only difference between them really is the name because both end up doing the same thing with the same basic attributes.

The real Tenifer process its self can't be used in the U.S. as the EPA doesn't allow it because in the Tennifer process cyanide salt is used, but parts finished with Tenifer can be used in the U.S. (for example, Glocks sold with Tenifer finishes). The Tenifer process could be used in the U.S. if the cyanide salt was replaced with a different chemical, and some companies have done so (Glock did in their Georgia manufacturing) and its not really the true Tenifer finish but just as tough and close enough to be legally called Tenifer.

Melonite is used on the S&W Sport. S&W chose Melonite for several reasons, two main reasons which are its main attributes are durability and it can be used on both 4140 steel and 416 stainless steel. However, on 416 stainless the process removes some of the material attributes which ultimately allows for rust and/or corrosion. Accuracy is not a selection factor for Melonite, its got nothing to do with accuracy really.