I plan on changing my A2 stock to a postban CAR stock on my 9mm AR.  I am planning on getting the post ban CAR stock from RRA or bushy.  The question is what kind of buffer/spring do i need? A regular full length buffer/spring or the CAR buffer/spring? because i am not sure about tube length of the post CAR.  Is it a preban Car pinned, or a different design that has a A2 length tube?

I already have a pre-ban CAR stock, could i just pin it myself.  Then i know which buffer to use.

its not legal to pin a car stock, you need the batf approved, "fixed stock", they are not actually fixed open, there is no way to close them, they have a full length buffer tube, and yes you use the full size spring and buffer from an a2 stock