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Hi Austin, S&W purports this to be "Mil-Spec".

according to S&W its "mil-Spec"

Austin, almost every civilian version of the AR is made to Mil-Spec as the design plans are in the public domain and are free to use royalty free. That said, many manufacturers may add their own design elements to the basic design. I will not go thru all the variations, but the most common one is on the lower priced ARs. The stock, buffer tube and spring are what is termed "commercial grade". The difference is the diameter of the buffer tube and spring are slightly smaller than the Mil Spec version, although the commercial tube will screw into the Mil-Spec lower receiver just fine. The S&W is unit you cited is a full Mil-Spec compliant lower assembly. I have purchased and redone a couple of very low cost DPMS Oracle AR-15s that come with commercial buffer tubes. A replacement Mil-Spec sized buffer tube and spring is around $20 and easy to replace. In one of them I used a flat wire spring rather than the traditional round wire type, and replaced all the furniture with Magpul items, I'm