Quote Originally Posted by jim View Post
I would say it's the upper, especially if it is a Square marked unit. They require a little finesse to get it in Seen 4 of them with different mfg names on them, still the square forge mark.
Nah. That's what I thought at first too. But the upper fits nicely on other lowers and I put two other uppers on the Arero lower and they all get cocked as soon as you get the pivot pin pushed through.