I am about to add another M4 to my collection. I want to know what the real difference besides the 4150 barrel is between these 2 rifles . The S&W X model is Smith's near top of the line M4 with factory Troy quad rail and Troy back up sighs. About the nicest Smith. VS a Standard Daniel Defense M4 that is a couple hundred higher. Besides the 4150 vs 4140 barrels what is the real difference from you experts? Wanna kill these ads? We can help!