Quoted:
I remember reading about people un-pin their post ban CAR stock, adjust it to a desirable length then re-pin it. You mean that's illegal?
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MOST, post-ban Non collapsing CAR stocks are a full length buffer tube and you couldn't break "the pin" and make them shorter. With these stocks the plastic stock is glued to the end of a CAR shaped full length buffer and short of cutting the stock off, shortening the buffer tube, welding a new cap on the buffer tube, replacing the buffer and buffer spring for CAR units (post-ban stocks come with rifle buffers and springs), and then you may need to mill new catches into the shortened buffer tube.... well, I just don't see people as being likely to want to do that much work to convert a $50 used item!
In answer to the question, you need to decide for yourself on what a "folding stock" is... The BATFE has said that a collapsing stock is a folding stock BUT there is no guidance beyond that as to what is needed to make it "non-folding".
With AKMS (AK-47 folding stocks) guys use a simple tac weld that could be easily ground off and the stock refinished later if the law dies without replacement so I assume you could apply the same logic to the AR-15 CAR stock. The problem with the plastic stock is there is no easy way to weld her in a specific position. You might try eppoxy as your liability is much less then a manufacturer who needed to make sure a person couln't easily modify their stock to an illegal form.
You might be able to do it with a metal stock as well but I am not sure it is worth the effort at this time. If you did, I would argue that the tac weld could be adequate as this is how other weapons get around the issue.
Personally, I would look for a cheap used Post-Ban CAR stock or use what you have until you see what the bill does. You may want to have a CAR stock in the safe at home, however, just in case the Crime Bill sunsets for a brief period before some new law comes into effect.
Good Luck!
QB