Ok..I'm building an XM177 clone and I just got in a A1 upper that had a circle FA on it. I have a new Tear Drop AF to put in it.

I drove out the roll pin, put the spring on the Tear Drop, inserted it into the reciever and drove the roll pin back in.

When I let the FA go, it popped out, The pin will no engage the Tear Drop but will engage the Cricle one.

Is there somehting I'm missing?

take out both mechanisms and compare them.
It sounds to me as if the notch on the teardrop  forward assist pawl that is held by the roll pin, is deformed or shallow. That would account for the unit not staying put after the pin is driven back through the FA housing.

Also, did you hold the FA unit in against the spring pressure when you reinstalled the roll pin?
If not, the notch I am referring to would not have been engaged.

HTH

malysh is correct.  The new forward assist has to be depressed about halfway in for the roll pin to engage the recess in the forward assist.

If not pressed in far enough, the forward assist falls out.  Pressed in too far, and the forward assist sticks.  Have to find that happy medium!

We want pics when done, too!  

I did press it in when re-installing the rol pin...

I can insert it all the way in with the roll pin in...it's like the roll pin is not big enough to hold the Tear Drop in..