From Brownell's / Magpul: SPECS: Reinforced molded polymer, Black, Dark Earth, O.D. Green, or Foliage Green. 7¼" (cm) OAL, 5¼" (13.3cm) high. Mil-Spec fits mil-spec 1.14" O.D. carbine buffer tubes. Universal fits rifles with 1.17” O.D. commercial AR-15 carbine buffer tubes. They have a "Universal" in stock but I honestly have no idea the difference between the two sizes. What's the overall impact here?