Terry Dyer use to do this on the MP5 stocks, worked well for smaller or younger shooters (friend's son found it a little too short, now he is six and a half feet with no issues ;-). Only down side is you have to hold the latch as will stop at each notch.
That kind of stock exists. When we upgraded our G3's, in 2007, they came with a 3-pos retractable stock. The added notch was a few centimeters shorter than fully extended, to make shooting with BA easier.
I assume that the stock is factory HK, or produced by a manufacturer in Norway, seeing as just modifying the existing legs didn't work. Stocks would break during shooting.
I found a way to make the A3 butt stock much more comfortable. I use a AMD-65 (AK variant) butt pad. You have to remove the glued on hard rubber pad from the A3, which is easy to do. I used a piece from a rubber door stop to keep the new pad from rocking and to keep it vertical. The supplied velcro strap is a bit too short and best to use something longer. The pad sits nicely on the shoulder and has plenty of contact area. Here is an example of one: http://centerfiresystems.com/PADAKA02.aspx
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