Agree with the above.
Even if you don't buy their stocks, Ironwood design talks about this on their website (or, at least, used to)
I added some OAL by using one of the aftermarket rubber butt-stocks too. AK-Builder.com has a couple of options. Ended up looking like this:
(That's an Amazon image. I'll try to find the bumper I used, but it has no texture on it)
And you can always dismount it to get back to the length you started with.