Out of the box the buttstock can be locked in two positions; fully extended or fully collapsed.
While this doesn't really bother me, it would have been cool if there were various stages like with the Raider stock. Being the curious fellow that I am, I set about giving the RS stock a variety of adjustment options. Turns out it's pretty simple to do.

The stock can be removed very easily once you have the blaster open. The blue collar contains the locking mechanism and allows the stock to slide in and out.

This long white piece is the bit that contains the locks so this is what we need to get at. The blue collar is all one piece so in order to get it off it will have to slide off the end. This is not currently possible because there are four 'lugs' on the grey shaft. You first need to slide the blue collar all the way to the back of the stock. By simply shaving the lugs off and creating a ramp, you can easily pull the blue collar forward and off of the stock.

Now the long white piece can be removed from the blue collar. You will see that there is a square hole at each end. These holes locate the mechanism in the stock and are what causes the stock to lock in place. By simply adding more holes you can add extra levels of adjustment to your buttstock. I chose to add one extra hole but there is no reason why you couldn't add more, just be careful to get the holes in line with the ones that are already there.

Replace the white piece and slide the blue collar back on to the stock and you're all done. No need to worry about pulling too hard and pulling the stock off during battle, the metal bar that passes through the stock will stop that.