Sig Sauer 556, this weapon is also known as the SCMR, basically it has a non collapsible stock and a bullet button on it so as to be communist territory california legal. If the buyer is living in the free country I will provide the parts to go back to a regular magazine release button.Otherwise this is one of the best weapons out there, 2 stage gas piston. I bought it new and put 10 rounds through it.Have all documentation and case. THE ACOG IS NOT INCLUDED however if someone wants a combo deal we can negotiate a price. The cat isnt included either.Selling as Im just used to the direct gas impingement AR platform and all parts and pieces ive stocked are not compatible with this weapon.