Hi everyone, I'm buying a 10" Sig 516 Upper (barrel, BCG, Charging handle, handgrips, etc.) from a private seller. It does not come with the "Sig heavy buffer and spring." I already have an AR-15 Pistol Lower (complete with the Tailhook Mod 2 brace). Do I need to buy the Sig heavy buffer and...

Hi all! New member & Sig 716 owner. I'm wondering if anyone has hands on experience with putting a 516 upper on a 716 lower. I've read that the inverse (716 upper on 516 lower) should work, but does that automatically mean both rifles are completely interchangeable (obviously with different...