Zombie post, I know, but for a reason.
I bit the bullet, and pinned an M4 stock for this lower.
I used a total of 4 roll pins:
Two on the left side of the lower "rail" on the receiver extension, one on the right, and one right through the adjustment pin it's self,
My logic was simple: If the pins in the sides fail for some reason, the pin that holds the stock in place still won't be able to move, and that will keep it completely fixed.
All of the pins going into the rail go through the plastic, through one side of the rail, and bottom out in a dimple on the inside channel on the opposite side of the rail. The pin going through the adjustment pin goes through the side of the stock, though both sides of the rail, and bottoms out on the other side of the plastic..
As for the epoxy, I went with
3m Scotch-Weld 460 epoxy. It has a 5000lbf/in shear strength, and a 60lbf/in peel strength. In short, it ain't comin' off.
I did all of my drilling before applying the epoxy.
I picked my pin locations, drilled though the plastic shell, then installed the shell on the extension. Then I used the holes in the plastic as guides for the holes in the metal, putting a small dimple in each location. I then removed the shell, and drilled though one wall of the extension rail, being careful to just dimple the other side of the rail to give each pin two points of support in the metal. By not drilling all the way through the extension rail, there's no way to just knock out the pins, and drive them through the plastic.
Once the drilling was done, I smeared the epoxy on the receiver extension, and the inside of the shell. As I slid the shell on, I wiped off the extra epoxy that built up at the front. I wanted to make sure there was a complete bond between the two parts for the entire length of the extension, so I made sure there was extra epoxy inside the shell to me smeared back.
The only down side of this was that some of the epoxy was pushed back into the shell, and I couldn't clean it out before it dried.
I also added a touch of epoxy under the little adjustment lever, mostly because the little rattle it made was annoying, and since it was now useless, there was no reason to put up with it any more.
All in all, I am pretty damn sure that you will be able to collapse the receiver extension tube LONG before you get that stock to move an even a fraction of an inch