The bottom front of the receiver extension should be over the outer lip of the buffer retention pin, and almost touching the inner post with the butt stock assembly installed correctly.  Secondary will be the top of the tube, and you may need to file the very topward edges of the tube down so they sit flush with the upper contours of the lower receiver inner threaded section.
(Read when you close the upper down onto the lower, the back of the carrier pushes the buffer slightly back off the buffer retainer pin, and if the top lip of the buffy tube is protruding past the inner back of the receiver, the carrier will bind on this lip on the way down).

Regarding the double feeds (two live rounds), you have mags that are in question.  With the shorter receiver extension and lighter buffer, the impact of the buffer off the back of the receiver extension during cycle is stronger (more of a jolt), and this impact is what will cause the rounds to be sprayed out of a questionable mag more-so then if the rig was sporting the A-2 stock.

What Gamma762 and Dano523 said. Back that sucker out one turn.


Another thought about the double-feeds; You did use the new CAR (shorter) spring, right?

double post.

Mike,
He may or may not be able to spin the receiver extension out one turn with it still retaining the buffer retainer pin, and I have the feeling that the end of his receiver extension was not correctly produced.

Here is a photo of a correctly produced receiver extension.
Note how the end is milled longer at the bottom to retain the buffer retain pin, and shorter on the top to follow/flush out with the back of the upper receiver edge.


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