ATF has withheld "post-ban" approval for stocks that started out as working collapsable stocks.  All of the fake ones that you see are legal because the buffer tubes are full-length and the machining for the stock positions wasn't done, in addition to pinning the stock in place.  What I'm saying is: ATF's legal opinion is that pinning a working telestock does not make it post-ban compliant.

But to answer your question, you would need a "carbine-length" buffer and spring.

-Troy